V4L/DVB (6208): pvrusb2: Implement programmatic means to extract prom contents

The pvrusb2 driver already has a method for extracting the FX2's
program memory back out to a user application; this ability is used to
facilitate manual firmware extraction as per the procedure documented
on the pvrusb2 web site.  This change follows that pattern and
implements a corresponding method to grab the binary contents of the
PVR USB2 prom (which for PVR USB2 devices can contain information in
addition to the usual Hauppauge metadata).

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