Description of problem:
When the net plugin is set to scan devices and apply net queues configuration
it can throw an unhandeld exception if the device has a combined channel with the n/a value. 


Version-Release number of selected component (if applicable):
OCP: 4.8.0-fc.9
CoreOS 8.4 GA
kernel-core 4.18.0 305.3.1.el8_4

How reproducible:
Always

Steps to Reproduce:
1. on an environment that has a non virtual net device
create a tuned profile with net plugin set:

[net]
channels=combined 1
2. apply the profile

Actual results:
An unhandled exception is thrown:

2021-06-16 13:30:21,578 ERROR    tuned.units.manager: BUG: Unhandled exception in start_tuning: invalid literal for int() with base 10: 'n/a'
2021-06-16 13:30:21,578 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 261, in instance_apply_tuning
    self._instance_apply_static(instance)
  File "/usr/lib/python3.6/site-packages/tuned/plugins/base.py", line 318, 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 441, 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 502, 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 468, in _channels
    return self._custom_parameters("channels", start, value, device, verify)
  File "/usr/lib/python3.6/site-packages/tuned/plugins/plugin_net.py", line 423, in _custom_parameters
    dev_params = params_current)
  File "/usr/lib/python3.6/site-packages/tuned/plugins/plugin_net.py", line 400, in _set_device_parameters
    if context == "channels" and (int(dev_params[next(iter(d))]) == 0 or str(dev_params[next(iter(d))]) == 'n/a'):
ValueError: invalid literal for int() with base 10: 'n/a'

Expected results:
Setting the net device channel to the required value should either dismissed with a warning (if not supported for multi channels) or be set to the desired value.

Additional info:
Getting available devices on the machine example:

find /sys/class/net -type l -not -lname *virtual* -printf '%f\n'
eno3
eno1
ens1f0
eno4
eno2
ens1f1

a net device with n/a value for the channel:

# ethtool -l eno2
Channel parameters for eno2:
Pre-set maximums:
RX:		4
TX:		4
Other:		n/a
Combined:	n/a
Current hardware settings:
RX:		4
TX:		1
Other:		n/a
Combined:	n/a