PCI: Do not use bus number zero from EA capability

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: 2dbce5901179 ("PCI: Assign bus numbers present in EA capability for bridges")
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Subbaraya Sundeep <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Cc: [email protected]	# v5.2+

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