PCI: dwc: ep: Ensure proper iteration over outbound map windows

PCI: dwc: ep: Ensure proper iteration over outbound map windows

Most systems' PCIe outbound map windows have non-zero physical addresses,
but the possibility of encountering zero increased after following commit
("PCI: dwc: Use parent_bus_offset").

'ep->outbound_addr[n]', representing 'parent_bus_address', might be 0 on
some hardware, which trims high address bits through bus fabric before
sending to the PCIe controller.

Replace the iteration logic with 'for_each_set_bit()' to ensure only
allocated map windows are iterated when determining the ATU index from a
given address.

Link: https://lore.kernel.org/r/20250315201548.858189-12-helgaas@kernel.org
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>