PCI/MSI: pci-xgene-msi: Fix CPU hotplug registration handling

The conversion to the new hotplug state machine introduced a regression
where a successful hotplug registration would be treated as an error,
effectively disabling the MSI driver forever.

Fix it by doing the proper check on the return value.

Fixes: 9c248f8896e6 ("PCI/xgene-msi: Convert to hotplug state machine")
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Duc Dang <dhdang@apm.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: stable@vger.kernel.org