staging: rtl8723au: Move last utility function from osdep_service.c to os_intfs.c

No point carrying a dedicated file just for this small helper, so move
it to os_intfs.c and get rid of osdep_service.c

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>