mmc: bcm2835: Don't overwrite max frequency unconditionally

The optional DT parameter max-frequency could init the max bus frequency.
So take care of this, before setting the max bus frequency.

Fixes: 660fc733bd74 ("mmc: bcm2835: Add new driver for the sdhost controller.")
Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Cc: <stable@vger.kernel.org> # 4.12+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/bcm2835.c

@@ -1265,7 +1265,8 @@ static int bcm2835_add_host(struct bcm2835_host *host)
 	char pio_limit_string[20];
 	int ret;
 
-	mmc->f_max = host->max_clk;
+	if (!mmc->f_max || mmc->f_max > host->max_clk)
+		mmc->f_max = host->max_clk;
 	mmc->f_min = host->max_clk / SDCDIV_MAX_CDIV;
 
 	mmc->max_busy_timeout = ~0 / (mmc->f_max / 1000);