PCI/MSI: xgene: Remove msi_controller assignment

After 8d63bc7b ("PCI/MSI: pci-xgene-msi: Get rid of struct
msi_controller"), it is no longer required to assign msi_controller for
X-Gene PCIe host bridge to support MSI.

Remove this unnecessary code.  This also avoids a warning message ("failed
to enable MSI") during boot.

[bhelgaas: changelog]
Signed-off-by: Duc Dang <dhdang@apm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Tanmay Inamdar <tinamdar@apm.com>