[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>

drivers/media/dvb-frontends/drxk_hard.c

@@ -2628,7 +2628,7 @@ static int GetDVBTSignalToNoise(struct drxk_state *state,
 		/* log(x) x = (16bits + 16bits) << 15 ->32 bits  */
 		c = Log10Times100(SqrErrIQ);
 
-		iMER = a + b;
+		iMER = a + b - c;
 	}
 	*pSignalToNoise = iMER;