staging: unisys: Do not use 0 as the default bus root device number

I used 0 as the device id for the bus root, neglecting the fact that
device 0 is a valid id in Unisys's configuration.  Modify this to
use UINT_MAX instead as a unique number.

As fallout from this change it was noticed the bus_no and dev_no was not
defined the same way consistently.  Fix visorbus.h to use u32 there.  Fix
the resulting printk warning too.

Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>