efi: Correct comment on efi_memmap_alloc

[ Commit db01ea88 upstream ]

Returning zero means success now.

Signed-off-by: Liu Zixian <liuzixian4@huawei.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>