staging: comedi: pcl816: trigger sources were validated in (*do_cmdtest)

The trigger sources were already validataed in the (*do_cmdtest) before the
(*do_cmd) is called. Remove the unnecessary checks in pcl816_ai_cmd().

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>