V4L/DVB (12684): DVB: make DVB_MAX_ADAPTERS configurable

Support for more than 8 DVB devices is requested regularly so make it
a kconfig variable instead of a header define. Values in the range 4-32
are tested.

Signed-off-by: Janne Grunau <j@jannau.net>
Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>