staging: comedi: ni_labpc: introduce labpc_ai_wait_for_data()

Introduce a helper function to wait for the a/d conversion to
complete.

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>