x86, Calgary: Limit the max PHB number to 256

The x3950 family can have as many as 256 PCI buses in a single system, so
change the limits to the maximum.  Since there can only be 256 PCI buses in one
domain, we no longer need the BUG_ON check.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
LKML-Reference: <20100701004519.GQ15515@tux1.beaverton.ibm.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>