rval wasn't set, resulting in probe returning zero instead of an error.

Fixes: de10c161 ("[media] smiapp: Get clock rate if it's not available through DT")
Reported-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>