[media] au8522: fix lock detection to be more reliable

Only looking at the lock register causes the status to float
between locked and not locked when there is no signal.  So improve
the logic to also examine the state of the FSC PLL, which results
in the lock status being consistently reported.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>