efi_loader: Fix return value for efi_add_runtime_mmio

The efi_add_runtime_mmio function incorrectly returned the added
address as return value rather than EFI_SUCCESS. Fix it by checking
the return value of efi_add_memory_map properly.

Fixes: f057cfef5dc ("efi_loader: exit status for efi_reset_system_init")
Signed-off-by: Alexander Graf <agraf@suse.de>