dm: mmc: Convert sdhci to support CONFIG_BLK

Update sdhci.c so that it works with driver model enabled for block devices.
Signed-off-by: NSimon Glass <sjg@chromium.org>

drivers/mmc/sdhci.c

@@ -137,7 +137,7 @@ static int sdhci_send_command(struct mmc *mmc, struct mmc_cmd *cmd,
 	int trans_bytes = 0, is_aligned = 1;
 	u32 mask, flags, mode;
 	unsigned int time = 0, start_addr = 0;
-	int mmc_dev = mmc->block_dev.devnum;
+	int mmc_dev = mmc_get_blk_desc(mmc)->devnum;
 	unsigned start = get_timer(0);
 
 	/* Timeout unit - ms */