media: omap3isp: Fix missing unlock in isp_subdev_notifier_complete()

Add the missing unlock before return from function
isp_subdev_notifier_complete() in the init error
handling case.

Fixes: ba689d93 ("media: omap3isp: Acquire graph mutex for graph traversal")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>