V4L/DVB (8896): pvrusb2: Implement crop support

Implement pvrusb2 driver plumbing to support cropping.  Submitted by a
pvrusb2 user.

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