mmc: avoid division by zero in meson_mmc_config_clock

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>
Gbp-Pq: Topic odroid-c2
Gbp-Pq: Name 0001-mmc-avoid-division-by-zero-in-meson_mmc_config_clock.patch