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>