tsnep: Fix ethtool channels

[ Upstream commit a7f99195 ]

According to the NAPI documentation networking/napi.rst, for the ethtool
API a channel is a IRQ/NAPI which services queues of a given type.

tsnep uses a single IRQ/NAPI instance for every TX/RX queue pair.
Therefore, combined channels shall be returned instead of separate tx/rx
channels.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>