[media] media/radio/wl1273: fix build errors

RADIO_WL1273 needs to make sure that the mfd core is built to avoid
build errors:

ERROR: "mfd_add_devices" [drivers/mfd/wl1273-core.ko] undefined!
ERROR: "mfd_remove_devices" [drivers/mfd/wl1273-core.ko] undefined!

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Matti Aaltonen <matti.j.aaltonen@nokia.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>