staging: comedi: skel: use cfc_check_trigger_arg_*() helpers

Use the new helpers in the step 3 tests of skel_ai_cmdtest().

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>