V4L/DVB (4376): Make it possible for run-time calculation of control min/max in pvrusb2

The internal control implementation in the pvrusb2 driver normally
encodes integer range limits using literal values in a const
structure.  This change adds two function pointers, which if not null
will be called through in order to determine integer min / max
values.

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