V4L/DVB (4176): Bug-fix: Fix memory overflow

Do not overflow kernel memory when more than BT878_MAX devices are 
present.

Signed-off-by: Sigmund Augdal Helberg <sigmund@snap.tv>
Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>