staging: comedi: usbduxsigma: remove usbduxsigma_attach

This driver originally used the 'attach' method in order to get
the firmware for the device from the comedi_config utility. It
now uses request_firmware_nowait() in the usb_driver probe to
get this firmware.

Since this driver has an 'attach_usb' method in the comedi_driver,
the 'attach' method can be removed because it is now optional.

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>