V4L/DVB: bttv-driver: document functions using mutex_lock

There are a few ancillary static routines used by ioctl functions
that takes bttv lock internally. As we'll be adding the same lock
for all ioctl's that need, we need to properly document them, to
avoid doing double locks

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