staging: comedi: adv_pci1710: absorb pci171x_ai_docmd_and_mode()

This helper function is called by the ai (*do_cmd) to setup and
start the async command based on the "mode" of operation. This
"mode" is determined by the cmd convert_src and start_src.

Move this "mode" determination and combine the functions.

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>