[U-Boot] efi_loader: Fix return value for efi_add_runtime_mmio

[Alexander Graf]

> 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 at suse.de>

Thanks, applied to efi-next

Alex