[media] stv0900: avoid going past array

Fix the following smatch warnings:
	drivers/media/dvb-frontends/stv0900_core.c:1183 stv0900_get_optim_carr_loop() error: buffer overflow 'cllas2' 11 <= 13
	drivers/media/dvb-frontends/stv0900_core.c:1185 stv0900_get_optim_carr_loop() error: buffer overflow 'cllas2' 11 <= 13
	drivers/media/dvb-frontends/stv0900_core.c:1187 stv0900_get_optim_carr_loop() error: buffer overflow 'cllas2' 11 <= 13
	drivers/media/dvb-frontends/stv0900_core.c:1189 stv0900_get_optim_carr_loop() error: buffer overflow 'cllas2' 11 <= 13
	drivers/media/dvb-frontends/stv0900_core.c:1191 stv0900_get_optim_carr_loop() error: buffer overflow 'cllas2' 11 <= 13

Reviewed-by: Michael Ira Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>