media: dvbdev.h: do some kernel-doc cleanups

Some kernel-doc warnings in <media/dvbdev.h> were introduced. A fixup
patch addressed them was already merged, but Randy's approach from:
https://lore.kernel.org/linux-media/20221203060931.19953-1-rdunlap@infradead.org



Had some advantages, as it moves the @dvbdev to the right place inside
dvb_remove_device() documentation and it makes clearer about what
refcounter struct dvb_device refers to.

So, apply the changes suggested by Randy.

Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>