media: staging: rkisp1: cap: fix return values from pm functions

If no errors occurs, pm functions return usage counters, so they can
return positive numbers.
This happens when streaming from multiple capture devices (mainpath and
selfpath).

Fix simultaneous streaming from mainpath and selfpath by not failing
when pm usage counters returns a positive number.

Signed-off-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>