[media] vivid: SDR cap: add control for FM deviation

Add user control to adjust generated FM deviation.
Default it to 75kHz like public FM radio broadcast.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>