staging: comedi: usbduxsigma: factor usb buffer allocation out of (*probe)

To make pushing the usb_driver (*probe) into the comedi_driver (*auto_attach)
cleaner, factor the usb buffer allocation out of the (*probe). This also
cleans up the failure paths in the (*probe).

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>