staging: comedi: adv_pci1724: use comedi_timeout() to wait for DAC idle state

Use the comedi_timeout() helper to wait for the DAC to be idle before
writing to it.

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>