[media] tw686x: Don't go past array

Depending on the compiler version, currently it produces the
following warnings:
	tw686x-video.c: In function 'tw686x_video_init':
	tw686x-video.c:65:543: warning: array subscript is above array bounds [-Warray-bounds]

This is actually bogus with the current code, as it currently
hardcodes the framerate to 30 frames/sec, however a potential
use after the array size could happen when the driver adds support
for setting the framerate. So, fix it.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>