mmc: Register only the first MMC device on MMC_TINY

When MMC_TINY is enabled, support for only one MMC device
is provided. Boards that register more than one device,
will just write over mmc_static keeping only the last one
registered.

This commit prevents this, keeping only the first MMC
device created. A debug warning message is added, if nothing
else, as a hint/documentation for developers.
Signed-off-by: NEzequiel Garcia <ezequiel@collabora.com>

drivers/mmc/mmc_legacy.c

@@ -150,6 +150,15 @@ struct mmc *mmc_create(const struct mmc_config *cfg, void *priv)
 {
 	struct mmc *mmc = &mmc_static;
 
+	/* First MMC device registered, fail to register a new one.
+	 * Given users are not expecting this to fail, instead
+	 * of failing let's just return the only MMC device
+	 */
+	if (mmc->cfg) {
+		debug("Warning: MMC_TINY doesn't support multiple MMC devices\n");
+		return mmc;
+	}
+
 	mmc->cfg = cfg;
 	mmc->priv = priv;