mmc: sdhci-esdhc-imx: correct pre_div for imx6q

According to spec, the pre_div for imx6q should be 1, or the biggest
clock rate we can get is a half of host clock rate.  This may cause
we can not get the proper clock rate as we want.  e.g. if the desired
clock is 200Mhz, however, the host clock is 200Mhz too, then it causes
the actual clock we get is 100Mhz due to pre_div is 2.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>