mmc: avoid division by zero in meson_mmc_config_clock

The Odroid C2 fails to read from mmc with U-Boot v2018.03.
The change avoids a division by zero.

The fix was suggested by Jaehoon in
https://lists.denx.de/pipermail/u-boot/2018-January/318577.html

Reported-by: Vagrant Cascadian <vagrant@debian.org>
Suggested-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Vagrant Cascadian <vagrant@debian.org>

drivers/mmc/meson_gx_mmc.c

@@ -35,6 +35,9 @@ static void meson_mmc_config_clock(struct mmc *mmc)
 	uint32_t meson_mmc_clk = 0;
 	unsigned int clk, clk_src, clk_div;
 
+	if (!mmc->clock)
+		return;
+
 	/* 1GHz / CLK_MAX_DIV = 15,9 MHz */
 	if (mmc->clock > 16000000) {
 		clk = SD_EMMC_CLKSRC_DIV2;