[media] cx22700: Fix potential buffer overflow

As new FEC types were added, we need a check to avoid overflows:
	drivers/media/dvb-frontends/cx22700.c:172 cx22700_set_tps() error: buffer overflow 'fec_tab' 6 <= 6
	drivers/media/dvb-frontends/cx22700.c:173 cx22700_set_tps() error: buffer overflow 'fec_tab' 6 <= 6

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