mpc83xx: PCIe: Don't start bus enumeration at 0

mpc83xx: PCIe: Don't start bus enumeration at 0

Currently we assign first_busno = 0 for the first PCIe hose, but this
scheme won't work if we have ordinary PCI hose already registered (its
first_busno value is 0 too).

The old code worked fine only because we have PCI disabled on
MPC837XEMDS boards in stand-alone mode (see commit 00f7bbae92e3b13f2b3
"mpc83xx: fix PCI scan hang on the standalone MPC837xE-MDS boards").
But on MPC837XERDB boards we have PCI and PCIe, so the bug actually
triggers.

So, to fix the issue, we should use pci_last_busno() + 1 for the
first_busno (i.e. last available busno).

Reported-by: Huang Changming <Chang-Ming.Huang@freescale.com>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>