V4L/DVB (4192): Use control helpers for saa7115, cx25840, msp3400.

Replace hardcoded control description by the standard ones supplied
by v4l2-common.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>