staging: comedi: usbduxsigma: move firmware request/upload into (*auto_attach)

The last step the usb_driver (*probe) does before handing off to the
comedi_driver (*auto_attach) is requesting and uploading the firmware.

Move the request/upload into the (*auto_attach) so we can use the
comedi_load_firmware() helper.

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>