Staging: comedi: add #include <linux/io.h> to a bunch of drivers

On some arches the function virt_to_bus() wasn't being pulled in due to
include chains being different.  So, as we are explicitly calling this
function, explicitly include the proper header file so all will build
properly.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>