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

Use the new helpers in the step 3 tests of usbdux_{ai,ao}_cmdtest().

Also, remove some dev_dbg() noise.

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>