net/mlx5e: Fix null pointer access when setting MTU of vport representor

net/mlx5e: Fix null pointer access when setting MTU of vport representor

MTU helper function is used by both conventional mlx5e
instances (PF/VF) and the eswitch representors. The representor
shouldn't change the nic vport context MTU, the VF is responsible for
that. Therefore set_mtu_cb has a null value when changing the
representor MTU.

Fixes: 10cf553ea706 ("net/mlx5e: Support configurable MTU for vport representors")
Signed-off-by: Adi Nissim <adin@mellanox.com>
Reviewed-by: Yevgeny Kliteynik <kliteyn@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>