media: staging/ipu7: fix isys device runtime PM usage in firmware closing

The PM usage counter of isys was bumped up when start camera stream
(opening firmware) but it was not dropped after stream stop(closing
firmware), it forbids system fail to suspend due to the wrong PM state
of ISYS. This patch drop the PM usage counter in firmware close to fix
it.

Cc: Stable@vger.kernel.org
Fixes: a516d36b ("media: staging/ipu7: add IPU7 input system device driver")
Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>