Those ancillary functions could be called even when compiled
without V4L2 support, as warned by ktest build robot:

All errors (new ones prefixed by >>):

>> ERROR: "__v4l2_mc_usb_media_device_init" [drivers/media/usb/dvb-usb/dvb-usb.ko] undefined!
>> ERROR: "__v4l2_mc_usb_media_device_init" [drivers/media/usb/dvb-usb-v2/dvb_usb_v2.ko] undefined!
>> ERROR: "__v4l2_mc_usb_media_device_init" [drivers/media/usb/au0828/au0828.ko] undefined!

Also, there's nothing there that are specific to V4L2. So, move
those ancillary functions to MC core.

No functional changes. Just function rename.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>