V4L/DVB: radio-am800: let v4l2_device_(un)register handle usb_get/set_intfdata

v4l2_device_register already sets the usb intfdata to v4l2_dev. So use
that instead.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>