V4L/DVB (7538): Adds selectable adapter numbers as per module option

The adapter_nr module options can be used to allocate static adapter
numbers on a driver level. It avoids problems with changing DVB apapter
numbers after warm/cold boot or device unplugging and repluging.

Each driver holds DVB_MAX_ADAPTER long array of the preferred order of
adapter numbers.

options dvb-usb-dib0700 adapter_nr=7,6,5,4,3,2,1,0 would result in a
reversed allocation of adapter numbers.

With adapter_nr=2,5 it tries first to get adapter number 2 and 5. If
both are already in use it will allocate the lowest free adapter number.

Signed-off-by: Janne Grunau <janne-dvb@grunau.be>
Acked-by: Hermann Pitton <hermann.pitton@arcor.de>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>