staging: comedi: dt9812: use comedi_to_usb_dev()

Use the helper function to get a usb_device pointer from a
comedi_device pointer. This removes the need for also getting
the usb_interface pointer in some cases.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>