IB/core: Search GID only for IB link layer

Even though API is only used by IPoIB driver, its incorrect to refer
RoCE GID table property to search for GID.

Look for only IB link layer to search for the GID.

Fixes: dbb12562

 ("IB/{core, ipoib}: Simplify ib_find_gid to search only for IB link layer")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>