media: cx24116: prevent overflows on SNR calculus

as reported by Coverity, if reading SNR registers fail, a negative
number will be returned, causing an underflow when reading SNR
registers.

Prevent that.

Fixes: 8953db7 ("V4L/DVB (9178): cx24116: Add module parameter to return SNR as ESNO.")
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/dvb-frontends/cx24116.c

@@ -741,6 +741,7 @@ static int cx24116_read_snr_pct(struct dvb_frontend *fe, u16 *snr)
 {
 	struct cx24116_state *state = fe->demodulator_priv;
 	u8 snr_reading;
+	int ret;
 	static const u32 snr_tab[] = { /* 10 x Table (rounded up) */
 		0x00000, 0x0199A, 0x03333, 0x04ccD, 0x06667,
 		0x08000, 0x0999A, 0x0b333, 0x0cccD, 0x0e667,
@@ -749,7 +750,11 @@ static int cx24116_read_snr_pct(struct dvb_frontend *fe, u16 *snr)
 
 	dprintk("%s()\n", __func__);
 
-	snr_reading = cx24116_readreg(state, CX24116_REG_QUALITY0);
+	ret = cx24116_readreg(state, CX24116_REG_QUALITY0);
+	if (ret  < 0)
+		return ret;
+
+	snr_reading = ret;
 
 	if (snr_reading >= 0xa0 /* 100% */)
 		*snr = 0xffff;