MMC core: limit minimum initialization frequency to 400kHz

Some controllers allow a much lower frequency than 400kHz.
Keep the minimum frequency within sensible limits.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

drivers/mmc/core/core.c

@@ -708,7 +708,13 @@ static void mmc_power_up(struct mmc_host *host)
 	 */
 	mmc_delay(10);
 
-	host->ios.clock = host->f_min;
+	if (host->f_min > 400000) {
+		pr_warning("%s: Minimum clock frequency too high for "
+				"identification mode\n", mmc_hostname(host));
+		host->ios.clock = host->f_min;
+	} else
+		host->ios.clock = 400000;
+
 	host->ios.power_mode = MMC_POWER_ON;
 	mmc_set_ios(host);