V4L/DVB (6548): pvrusb2: Fix oops on module removal

The pvrusb2 driver is tearing down its sysfs related pieces in the
incorrect order.  This leaves dangling pointers which causes the
kernel device core to oops.  The problem has been present virtually
forever but became malignant with the changeover to the way of
handling /sys/class.  Fix is just to make sure we don't tear down the
class structure until AFTER the driver instances are deregistered.

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