mmc: sdhci-iproc: fix oops in sdhci_iproc_writew

The driver co-allocates sdhci_iproc_host with sdhci_pltfm_host and so to
access it we need to use sdhci_pltfm_priv() and not pltfm_host->priv.

Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>