media: mb86a16: avoid division by zero

As warned by smatch:
	drivers/media/dvb-frontends/mb86a16.c:1690 mb86a16_read_ber() error: uninitialized symbol 'timer'.
	drivers/media/dvb-frontends/mb86a16.c:1706 mb86a16_read_ber() error: uninitialized symbol 'timer'.

There is a potential risk of doing a division by zero if
timer is not handled well. Enforce it by setting a bit mask
for the values used to select the timer.

It should be noticed that I don't have mb86a16 datasheet. So,
the bitmask was guessed based on the existing checks for
the field. At worse case scenario, it will just show a
badly calculated bit error rate, but it won't crash.

While here, optimize the logic to prevent uneeded tests.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/dvb-frontends/mb86a16.c

@@ -1677,15 +1677,15 @@ static int mb86a16_read_ber(struct dvb_frontend *fe, u32 *ber)
 			 * the deinterleaver output.
 			 * monitored BER is expressed as a 20 bit output in total
 			 */
-			ber_rst = ber_mon >> 3;
+			ber_rst = (ber_mon >> 3) & 0x03;
 			*ber = (((ber_msb << 8) | ber_mid) << 8) | ber_lsb;
 			if (ber_rst == 0)
 				timer =  12500000;
-			if (ber_rst == 1)
+			else if (ber_rst == 1)
 				timer =  25000000;
-			if (ber_rst == 2)
+			else if (ber_rst == 2)
 				timer =  50000000;
-			if (ber_rst == 3)
+			else /* ber_rst == 3 */
 				timer = 100000000;
 
 			*ber /= timer;
@@ -1697,11 +1697,11 @@ static int mb86a16_read_ber(struct dvb_frontend *fe, u32 *ber)
 			 * QPSK demodulator output.
 			 * monitored BER is expressed as a 24 bit output in total
 			 */
-			ber_tim = ber_mon >> 1;
+			ber_tim = (ber_mon >> 1) & 0x01;
 			*ber = (((ber_msb << 8) | ber_mid) << 8) | ber_lsb;
 			if (ber_tim == 0)
 				timer = 16;
-			if (ber_tim == 1)
+			else /* ber_tim == 1 */
 				timer = 24;
 
 			*ber /= 2 ^ timer;