PCI/TPH: Restore TPH Requester Enable correctly

PCI/TPH: Restore TPH Requester Enable correctly

When we reenable TPH after changing a Steering Tag value, we need the
actual TPH Requester Enable value, not the ST Mode (which only happens to
work out by chance for non-extended TPH in interrupt vector mode).

Link: https://lore.kernel.org/r/13118098116d7bce07aa20b8c52e28c7d1847246.1738759933.git.robin.murphy@arm.com
Fixes: d2e8a34876ce ("PCI/TPH: Add Steering Tag support")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Wei Huang <wei.huang2@amd.com>