[media] drxk: Fix bogus signal strength indicator

The DVBv3 signal strength indicator is bogus: it doesn't range
from 0 to 65535 as it would be expected. Also, 0 means the max
signal strength.
Now that a better way to estimate it was added, use the new
way.

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