x86/efistub: Fix PCI ROM preservation in mixed mode

preserve_pci_rom_image() was accessing the romsize field in
efi_pci_io_protocol_t directly instead of using the efi_table_attr()
helper. This prevents the ROM image from being saved correctly during a
mixed mode boot.

Fixes: 2c3625cb ("efi/x86: Fold __setup_efi_pci32() and __setup_efi_pci64() into one function")
Signed-off-by: Mikel Rychliski <mikel@mikelr.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>