PCI: Do not use bus number zero from EA capability

mainline inclusion
from mainline-v5.5-rc1
commit 73884a70
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I47H3V
CVE: NA

--------------------------------

As per PCIe r5.0, sec 7.8.5.2, fixed bus numbers of a bridge must be zero
when no function that uses EA is located behind it.  Hence, if EA supplies
bus numbers of zero, assign bus numbers normally.  A secondary bus can
never have a bus number of zero, so setting a bridge's Secondary Bus Number
to zero makes downstream devices unreachable.

[bhelgaas: retain bool return value so "zero is invalid" logic is local]
Fixes: 2dbce590 ("PCI: Assign bus numbers present in EA capability for bridges")
Link: https://lore.kernel.org/r/1572850664-9861-1-git-send-email-sundeep.lkml@gmail.com


Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org	# v5.2+
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Reviewed-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>