V4L/DVB: cx88-alsa: prevent out-of-range volume setting

Ensure that volume values are always in the allowed range.  Otherwise,
it would be possible to set other bits in the AUD_VOL_CTL register or to
get a wrong sign in the AUD_BAL_CTL register.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>