net/mlx5e: Don't allow more than max supported channels

Consider MLX5E_MAX_NUM_CHANNELS @ethtool set/get_channels

Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>