pci_rom: fix may be used uninitialized warning

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 55616b86c745fcac5a791268ab8e7cba36965c0f the ram variable.

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