[media] staging: v4l: omap4iss: Fix sub-device power management code

The same bug was present in the omap4iss driver as was in the omap3isp
driver. The code got copied to the omap4iss driver while broken. Fix the
omap4iss driver as well.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>