staging/comedi: use comedi_usb_driver_(un)register for init/deinit

Instead of coding calling comedi_driver_(un)register and
usb_(de)register directly we can use the comedi_usb_driver_(un)register
wrapper.
This removes some boilerplate and is less error prone.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>