staging: comedi: usbduxsigma: init 'pwm_buf_sz' in usbduxsigma_auto_attach()

Initialize this variable with then_ai_urbs and n_ao_urbs in the
attach. Then usbduxsigma_alloc_usb_buffers() can just check the
variable to determine if the urb needs to be allocated.

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>