pci_rom: fix may be used uninitialized warning

Building pci_rom.c with my toolchain complains about may be used uninitialized
rom varaible:

---8<---
+drivers/pci/pci_rom.c:269:25: note: 'rom' was declared here
w+drivers/pci/pci_rom.c: In function 'dm_pci_run_vga_bios':
w+drivers/pci/pci_rom.c:154:14: warning: 'rom' may be used uninitialized in this function [-Wmaybe-uninitialized]
--->8---

Fix this as done in 55616b86c7 the ram variable.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-by: Anatolij Gustschin <agust@denx.de>

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 = NULL;
+	struct pci_rom_header *rom = NULL, *ram = NULL;
 	int vesa_mode = -1;
 	bool emulate, alloced;
 	int ret;