V4L/DVB (9523): Increase number of BT8XX devices supported in a system

The BT8XX device driver currently only supports 16 such devices in a
system. This is too small for many surveillance applications. This
patch increases the number to 32.

Signed-off-by: Alan McIvor <alan.mcivor@reveal.co.nz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>