[media] vpx3220: signedness bug in vpx3220_fp_read()

The intent was to return -1 on error and that's what the callers expect
but the current code returns USHRT_MAX instead.

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>