[media] drxk: fix CNR calculus

Changeset 8f3741e0

 accidentally broke the CNR estimation. It should
be calculated as "a + b - c". However, previously, the subtraction
by c only occurred if SNR would be positive, due to a bad binding
to DVBv3 API.

This also fixes the following warning:
	drivers/media/dvb-frontends/drxk_hard.c:2556:6: warning: variable 'c' set but not used [-Wunused-but-set-variable]

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