V4L/DVB (7308): pvrusb2: Define digital control scheme device attributes

Unlike analog control, control of the digital side is not nearly as
uniform among different devices.  So we have to specify the correct
digital control scheme as a new device attribute.

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