staging: comedi: adl_pci6208: don't deadlock while waiting to write ao data

Remove a possible deadlock while waiting to write the analog output data.

The data transfer rate for every D/A data write in this driver is 2.2us.
Wait up to 10us for the board to be ready then and return -ETIME.

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>