[U-Boot] efi_loader: don't increment part twice per loop

[Alexander Graf]

> Correct a mistake in the part number handling of
> 16a73b249d138fedeb188710533902ed7aac1ddc and only increment part once
> per loop.
> 
> Signed-off-by: Jonathan Gray <jsg at jsg.id.au>
> Tested-by: Peter Robinson <pbrobinson at gmail.com>

Thanks, applied to efi-next

Alex