V4L/DVB (5702): Fix Kconfig items to avoid linkedition errors

The recent changes on Kconfig broke compilation when VIDEO_DEV is compiled
as module. On some cases, drivers like VIDEO_BUF are compiled with 'y' option
instead of 'm':

...

Thanks to: Toralf Forster <toralf.foerster@gmx.de> for pointing this issue.
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>