staging: comedi: adv_pci1710: use cfc_check_trigger_src

Use the helper function cfc_check_trigger_src for the "step 1"
tests of pci171x_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>