If get_pbus() is called for a device which has no parent/ancestor bus

If get_pbus() is called for a device which has no parent/ancestor bus
with nonzero PCI bus operations, get_pbus() will get stuck in a silent
endless loop.
Detect the endless loop and break out with an error message.

Such a situation can happen if the device tree is not yet
initialized/walked completely.

This fixes the unexplainable hang if pci_{read,write}_config{8,16,32}was
used in early mainboard code for the AMD DBM690T. Instead, the code will
now die() with a meaningful error message.

Thanks to Ward Vandewege for testing my patches to track down that bug.

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Marc Jones <marcj303@gmail.com>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@3972 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1