mmc: sdhci: Do not call implementations of mmc host ops directly

Drivers must be able to provide their own implementations for mmc host
operations. Consequently, SDHCI should call those not the default
implementations. Do that by calling indirectly through the mmc host ops
function pointers.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>