staging: comedi: ni_pcidio: Add comedi_poll support

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 the COMEDI_POLL ioctl to the ni_pcidio
driver.

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