V4L/DVB (5044): Pvrusb2: Allow overriding vbi and radio device minor numbers

Support specification of vbi and radio device minor numbers in a
manner similar to the video device minor number.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>