drivers/pci/pci_rom.c: fix compile warning under 64bit mode

Fix this:
drivers/pci/pci_rom.c:95:15: warning: cast to pointer from
integer of different size [-Wint-to-pointer-cast]
rom_header = (struct pci_rom_header *)rom_address;

Signed-off-by: Minghuan Lian <Minghuan.Lian@freescale.com>