mmc: sdhci-of-esdhc: Pre divider starts at 1

For PowerPC esdhc pre divider starts at 1, fixing the increases
the actual clock from 40KHz to 50 KHz.

Signed-off-by: Joakim Tjernlund <joakim.tjernlund@transmode.se>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>