[media] siano: fix signal strength and CNR stats measurements

There are a number of small issues with the stats refactoring:
	- InBandPwr better represents the signal strength;
	- Don't zero signal strength /cnr if no lock;
	- Fix signal strength/cnr scale;
	- Don't need to fill PER/BER if not locked, as the
	  code will disable those stats anyway.

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