mmc: fix division by zero in MMC core

The card is not always clocked and the clock frequency zero is perfectly
legal, thus this code in mmc_set_data_timeout() may cause a division by
zero. It will be triggered more often if you're using software clock
gating but can be triggered under other conditions too.
Reported-by: Pierre Tardy <tardyp@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Reviewed-by: Chris Ball <cjb@laptop.org>
Cc: <stable@kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>

drivers/mmc/core/core.c

@@ -299,8 +299,9 @@ void mmc_set_data_timeout(struct mmc_data *data, const struct mmc_card *card)
 		unsigned int timeout_us, limit_us;
 
 		timeout_us = data->timeout_ns / 1000;
-		timeout_us += data->timeout_clks * 1000 /
-			(mmc_host_clk_rate(card->host) / 1000);
+		if (mmc_host_clk_rate(card->host))
+			timeout_us += data->timeout_clks * 1000 /
+				(mmc_host_clk_rate(card->host) / 1000);
 
 		if (data->flags & MMC_DATA_WRITE)
 			/*