Fixes warning given by executing "make C=2 drivers/pci/"

Sparse output:
CHECK drivers/pci/controller/dwc/pcie-designware.c
 drivers/pci/controller/dwc/pcie-designware.c:432:52: warning:
 cast truncates bits from constant value (ffffffff7fffffff becomes
 7fffffff)

Link: https://lore.kernel.org/r/7ea7f7d342f97c758949a17b870012f52ce5b3f5.1600767645.git.gustavo.pimentel@synopsys.comReported-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Joao Pinto <jpinto@synopsys.com>