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

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

Note that cmd->stop_src only has one trigger source so the extra
test is not required.

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>