staging: comedi: pcmmio: return error if ai conversion times out

Rename adc_wait_ready() so it has namespace associated with the
driver.

Fix the function so it returns an errno if the conversion times
out. Propogate this errno if it happens.

Define the analog input status register to remove the magic numbers.

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>