[U-Boot] efi_loader: remove double EFI_EXIT() with efi_unsupported

[Alexander Graf]

> Probably this went unnoticed before, but it causes problems with
> addition of 804b1d73 ("efi_loader: log EFI return values too")
> 
> Signed-off-by: Rob Clark <robdclark at gmail.com>

Thanks, applied to efi-next

Alex