staging: comedi: usbdux: tidy up usbdux_alloc_usb_buffers()

Tidy up the error path by doing all the basic allocations then checking
for failures.

We have the comedi device now so fill in the urb->context.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>