net/mlx5: Remove impossible checks of interface state

The interface state is constant at this stage and checked
before calling to the register/unregister reserved GIDs.

There is no need to double check it.

Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>