tsnep: Add ethtool::get_channels support

Allow user space to read number of TX and RX queue. This is useful for
device dependent qdisc configurations like TAPRIO with hardware offload.
Also ethtool::get_per_queue_coalesce / set_per_queue_coalesce requires
that interface.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>