Staging: comedi: addi_common.h: properly mark this variable as __iomem

It's not an unsigned long, it's a __iomem pointer, so mark it as
such.

Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>