media: aspeed: refine hsync/vsync polarity setting logic

To prevent inaccurate detections of resolution, this commit enables
clearing of hsync/vsync polarity bits based on probed sync state.

Signed-off-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
Reviewed-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>