[media] av7110: potential divide by zero

"len" comes from dvb_video_ioctl() and there is a possibility that it is
zero.  We do a divide by len later in the function so that's not ok.

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>