IB/rxe: Use DEVICE_ATTR_RO macro to show parent field

Use DEVICE_ATTR RO() macro and rename the show function accordingly.

Signed-off-by: Kamal Heib <kamalh@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>