[media] si2168: relax demod lock checks a little

bit3 was not cleared always leaving driver reporting demod is not
fully locked. Do not check bit0 as it seems to be always 0.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>