[media] av7110: don't allow negative volumes

The issue here is that we there is a static checker warning because we
have a user controlled volume setting and we cap the upper bound but we
allow negative numbers.  Negative volumes don't make sense, so let's
make these variables unsigned.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>