net/mlx4_en: Remove unnecessary checks when setting num channels

Boundaries checks for the number of RX, TX, other and combined channels
should be checked by the caller and not in the driver.

In addition, remove wrong memset on get channels as it overrides the cmd
field in the requester struct.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>