ld*_dma() returns a MemTxResult type. Do not discard
it, return it to the caller.

Update the few callers.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211223115554.3155328-24-philmd@redhat.com>