PCI: designware: Use NULL instead of false

of_get_address() expects pointers in the third and fourth parameters.

Pass NULL in order to fix the following sparse warnings:

  drivers/pci/host/pcie-designware.c:433:51: warning: Using plain integer as NULL pointer
  drivers/pci/host/pcie-designware.c:433:58: warning: Using plain integer as NULL pointer

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Jingoo Han <jg1.han@samsung.com>