pci_rom.c: Fix may be used uninitialized warning

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>