pci_rom.c: Fix may be used uninitialized warning

With gcc-5.x we get:
drivers/pci/pci_rom.c: In function 'dm_pci_run_vga_bios':
drivers/pci/pci_rom.c:352:3: warning: 'ram' may be used uninitialized in
this function [-Wmaybe-uninitialized]

While unconvinced that this can happen in practice (if we malloc we set
alloced to true, it will be false otherwise), silence the compiler.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/pci/pci_rom.c

@@ -266,7 +266,7 @@ int dm_pci_run_vga_bios(struct udevice *dev, int (*int15_handler)(void),
 			int exec_method)
 {
 	struct pci_child_platdata *pplat = dev_get_parent_platdata(dev);
-	struct pci_rom_header *rom, *ram;
+	struct pci_rom_header *rom, *ram = NULL;
 	int vesa_mode = -1;
 	bool emulate, alloced;
 	int ret;