[media] tuner-xc2028: Fix signal strength report

There are several bugs at the signal strength algorithm:

	- It is using logical OR, instead of bit OR;
	- It doesn't wait up to 18 ms as it should;
	- the strength range is not ok.

Rework on it, in order to make it work.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>