IB/{core, ipoib}: Simplify ib_find_gid() for unused ndev

ib_find_gid() is only used by IPoIB driver. For IB link layer, GID table
entries are not based on netdevice. Netdevice parameter is unused here.
Therefore, it is removed.

Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>