IB/core: Reorder GID delete code for RoCE

Reorder GID delete code so that the driver del_gid operation is executed
before nullifying the gid attribute ndev parameter, this allows drivers
to access the ndev during their gid delete operation, which makes more
sense since they had access to it during the gid addition operation.

Signed-off-by: Patrisious Haddad <phaddad@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>