[media] em28xx: fix Terratec Grabby AC97 codec detection

EMP202 chip inside Terratec Grabby (hw rev 2) seems to require some time
before accessing reliably its registers. Otherwise it returns some values
previously put on the I2C bus.

To account for that period, we delay card setup until we have a proof that
accessing AC97 registers is reliable. We get this proof by polling
AC97_RESET until the expected value is read. We also check that unrelated
registers don't return the same value. This second check handles the case
where the expected value is constantly returned no matter which register
is accessed.

Signed-off-by: Matthieu Rogez <matthieu.rogez@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>