V4L/DVB (3964): Bt8xx/bttv-cards.c: fix off-by-one errors

This patch fixes two off-by-one errors spotted by the Coverity checker.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>