IB/providers: Avoid zero GID check for RoCE

Now that the IB core GID cache ensures that a zero GID doesn't exist in
the GID table remove zero GID checks from the provider drivers for
clarity.

Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>