mmc: sdhci-esdhc-imx: manually reset MIX_CTRL for usdhc

It's another violation to SDHC spec that software reset on usdhc
does not reset MIX_CTRL register.  Have to do it manually, otherwise
the preserving of the register bits (e.g. AC23EN) may cause mmc card
fail to be initialized.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Chris Ball <cjb@laptop.org>