V4L/DVB (9361): Dynamic DVB minor allocation

Implement dynamic minor allocation for DVB, to allow more than four
devices of the same type per adapter, based on drivers/usb/core/file.c.

Add a new config option, DVB_DYNAMIC_MINORS, to make use of this
feature, which defaults to no for backwards compatibility.

Signed-off-by: Andreas Oberritter <obi@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>