[media] DocBook: Merge FE_SET_PROPERTY/FE_GET_PROPERTY ioctl description

Instead of having two refentries, merge them into just one,
like what's done with other similar ioctls at V4L2 side.

That makes the entry cleaner and will allow to add the associated
structures together with the refentry.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>