[media] cx25840 / cx23885: Fixing audio/volume regression

Since the conversion to subdev in Oct 2010 the audio controls have
not functioned correctly in the cx23885 driver. Passing values of
0-3f did not translate into meaningfull register writes. I've
converted the cx23885 driver to match the cx25840 volume control
definition and now audio is working reliably again.

Signed-off-by: Steven Toth <stoth@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>