[media] pvrusb2: Remove unused variable

drivers/media/usb/pvrusb2/pvrusb2-v4l2.c: In function 'pvr2_v4l2_dev_init':
drivers/media/usb/pvrusb2/pvrusb2-v4l2.c:1268:21: warning: variable 'usbdev' set but not used [-Wunused-but-set-variable]

This warning is due to changeset a28fbd04, with removed the
usage of usbdev inside pvr2_v4l2_dev_init().

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Mike Isely <isely@pobox.com>