net/mlx4_en: Set number of rx/tx channels using ethtool

Add support to changing number of rx/tx channels using
ethtool ('ethtool -[lL]'). Where the number of tx channels specified in ethtool
is the number of rings per user priority - not total number of tx rings.

Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>