mmc: sdhci: Replace host->mmc with mmc where possible

After the switch to the MMC core regulator infrastucture, we already
have a local "mmc" pointer in various functions. There is no longer a
need to access the data structure via host->mmc.

Signed-off-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>