V4L/DVB (8898): pvrusb2: Be able to programmatically retrieve a control's default value

The pvrusb2 control mechanism up until now has used a constant int to
hold a control's default value.  This change makes it possible to
retrieve the control's default through some other means, e.g. as a
result of a query from lower level software.

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