staging: comedi: me4000: simplify ai_prepare()

The ai (*do_cmd_test) validates the trigger sources in Step 2b to ensure
that they are compatible. Save the 'ai_ctrl_mode' that will be used in the
private data so that ai_prepare(), which is called by the ai (*do_cmd),
does not have to recheck the sources in order to figure it out.

Also, tidy up the stop trigger checks so that the ME4000_AI_CTRL_HF_IRQ
bit is set in the common code path.

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>