[media] tw686x: avoid going past array

Fix those two warnings:
	drivers/media/pci/tw686x/tw686x-video.c:69 tw686x_fields_map() error: buffer overflow 'std_525_60' 31 <= 31
	drivers/media/pci/tw686x/tw686x-video.c:73 tw686x_fields_map() error: buffer overflow 'std_625_50' 26 <= 26

I had those changes at the last version of my patch, but I ended
by merging the previous version by mistake.

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