V4L/DVB (8900): pvrusb2: Implement cropping pass through

This builds upon the previous pvrusb2 change to more formally
implement full cropping support.  This enables access from the
driver's V4L interface, and enables access to full capabilities from
sysfs as well.  Note that this is only effective when in analog mode.
It also will only work when the underlying digitizer's driver (saa7115
or cx25840 depending on the hardware) also implements the appropriate
functions.

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