[media] cx25840: Prevent device probe failure due to volume control ERANGE error

This patch fixes a regression that crept into 2.6.36.

The volume control scale in the cx25840 driver has an unusual mapping
from register values to v4l2 volume control values.  Enforce the mapping
limits, so that the default volume control setting does not fall out of
bounds to prevent the cx25840 module device probe from failing.

Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>