Fix the following sparse warning:

drivers/pci/controller/dwc/pcie-intel-gw.c:456:5: warning: symbol
'intel_pcie_cpu_addr' was not declared. Should it be static?

Link: https://lore.kernel.org/r/20200415084953.6533-1-yanaijie@huawei.comReported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>