staging: comedi: addi_apci_3501: introduce apci3501_wait_for_dac()

Refactor the code that waits for the DAC to be ready into a helper
function.

A timeout of some sort should be added to this helper so code the
users to expect the error condition. In i_APCI3501_WriteAnalogOutput()
just return the error and don't actually write the new value to the
DAC. In apci3501_reset() output a dev_warn() that the DAC was not
ready.

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