staging: comedi: pcl816: introduce pcl816_ai_soft_trig()

Introduce a helper function to start a software triggered analog input
conversion.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>