Description of problem:
If tuned's net  plugin is configured with a list of devices such that the
first one returns ethtool error (Operation not supported), the plugin_net
terminates without parsing the rest of the listed network devices.

Version-Release number of selected component (if applicable):
tuned-2.15.0-2.el8.noarch

How reproducible:
Whenever we have a network device that doesn't support ethtool
configuration and there are other devices after that in the list that do
support ethtool configuration.

Steps to Reproduce:
1. Bring up a rhel8.4 host
2. Apply any tuned profile eg. realtime-virtual-host and configure the
   realtime/tuned.conf with something like:
   [net]
   type=net
   devices=br0,enp5s0f0
   channels=combined ${f:check_net_queue_count:${netdev_queue_count}}
   
   Note:
   To forcefully reproduce this issue I removed the check from tuned where
   it removes virtual devices from the list of network devices. This allows
   the virtual device to return  "Operation not supported" and the
   plugin_net script to fail.
   However, this issue should also be reproducible whenever we have a
   physical network device that doesn't support ethtool configuration.

3. Apply the profile and reboot or after the first reboot.

Actual results:
ERROR    tuned.utils.commands: Executing ethtool error: netlink error: Operation not supported
ERROR    tuned.utils.commands: Executing ethtool error: netlink error: Operation not supported
ERROR    tuned.units.manager: BUG: Unhandled exception in start_tuning: 'NoneType' object has no attribute 'items'
ERROR    tuned.units.manager: Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/tuned/units/manager.py", line 119, in _try_call
    return f(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/tuned/plugins/instance/instance.py", line 78, in apply_tuning
    self._plugin.instance_apply_tuning(self)
  File "/usr/lib/python3.6/site-packages/tuned/plugins/base.py", line 264, in instance_apply_tuning
    self._instance_apply_static(instance)
  File "/usr/lib/python3.6/site-packages/tuned/plugins/base.py", line 321, in _instance_apply_static
    self._execute_all_device_commands(instance, instance.assigned_devices)
  File "/usr/lib/python3.6/site-packages/tuned/plugins/base.py", line 446, in _execute_all_device_commands
    self._execute_device_command(instance, command, device, new_value)
  File "/usr/lib/python3.6/site-packages/tuned/plugins/base.py", line 507, in _execute_device_command
    command["custom"](True, new_value, device, False, False)
  File "/usr/lib/python3.6/site-packages/tuned/plugins/plugin_net.py", line 475, in _channels
    return self._custom_parameters("channels", start, value, device, verify)
  File "/usr/lib/python3.6/site-packages/tuned/plugins/plugin_net.py", line 436, in _custom_parameters
    in params_current.items()
AttributeError: 'NoneType' object has no attribute 'items'

Expected results:
Tuned should apply the network configuration to remaining devices that
supports it without terminating.

Additional info:
The issue is also reproducible upstream.