ethtool: add get/set channels functions

[sohorx]

Add simple setter/getter for ETHTOOL_CHANNELS

exemple:

>>> from pyroute2.ethtool import Ethtool
>>> with Ethtool() as tool:
...     print(tool.get_channels('eth0'))
...
EthtoolChannels(max_rx=0, max_tx=0, max_other=1, max_combined=2, rx_count=0, tx_count=0, other_count=1, combined_count=1)
>>> with Ethtool() as tool:
...     tool.set_channels('eth0', {'combined_count': 2})
...     print(tool.get_channels('eth0'))
...
EthtoolChannels(max_rx=0, max_tx=0, max_other=1, max_combined=2, rx_count=0, tx_count=0, other_count=1, combined_count=2)
>>>

Thoses fonctionnalities can be found with ethtool -l and ethtool -L

[svinota]

Thanks, merged!

PS: pls don't forget to run make format on the code while making PR, thanks!