staging: comedi: usbdux: move usb buffer allocation into new function

Move all the usb buffer allocation code in the usb_driver (*probe)
into a new function, usbdux_alloc_usb_buffers(). This allows tidying
up the error path 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>