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: f5de469 ("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>

Author: sergey-senozhatsky

drivers/mmc/host/mtk-sd.c

@@ -1492,6 +1492,7 @@ static void msdc_ops_request(struct mmc_host *mmc, struct mmc_request *mrq)
 	if (mrq->data) {
 		msdc_prepare_data(host, mrq->data);
 		if (!msdc_data_prepared(mrq->data)) {
+			host->mrq = NULL;
 			/*
 			 * Failed to prepare DMA area, fail fast before
 			 * starting any commands.