media: nuvoton: npcm-video: Fix stuck due to no video signal error

Fix the issue when start_frame and detect_resolution
functions are executed at the same time, which may cause driver
stops capturing due to status of no video signal error.

Signed-off-by: Michael Chang <zhang971090220@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>