mtk-sd: reset host->mrq on prepare_data() error

Do not leave host with dangling ->mrq pointer if we hit
the msdc_prepare_data() error out path.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Fixes: f5de4699 ("mtk-sd: Prevent memory corruption from DMA map failure")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250625052106.584905-1-senozhatsky@chromium.org


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>