staging: comedi: usbdux: use comedi_buf_read_samples()

Use comedi_buf_read_samples() to read the analog output samples from
the async buffer. That function will set the COMEDI_CB_BLOCK event
when samples are read from the async buffer.

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>