staging: comedi: usbduxsigma: use comedi_load_firmware()

Use comedi_load_firmware() instead of duplicating the code in a
private function.

Also, rename firmwareUpload() to have namespace associated with
the driver.

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>