powerpc/fsl-pci: Only scan PCI bus if configured as a host

We change fsl_add_bridge to return -ENODEV if the controller is working in
agent mode. Then check the return value of fsl_add_bridge to guarantee
that only successfully added host bus will be scanned.

Signed-off-by: Jia Hongtao <B38951@freescale.com>
Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>