pci: Avoid assigning PCI resources that are below 0x1000

commit b7598a43 ("[PATCH] Avoid assigning PCI resources from
zero address") only moved the bus lower address to 0x1000 if the
given bus start address is zero. The comment said 0x1000 is a
reasonable starting value, hence we'd better apply the same
adjustment when the given bus start address is below 0x1000.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>