staging: comedi: ni_pcidio: Support trailing edge external trigger

This patch is ported over by me (Ian Abbott) from the out-of-tree Comedi
git repository at "git://comedi.org/git/comedi/comedi.git

".

The original patch is by Nicholas Nell.

The patch adds support for trailing (falling) edge external triggers for
scans in asynchronous command support in the ni_pcidio driver.  This is
supported at least on the PCI-DIO_32HS and PCI-6533 boards; not sure
about the other boards.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: Nicholas Nell <nicholas.nell@colorado.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>