mmc: sdhci-iproc: use sdhci_pltfm_unregister directly

The sdhci_iproc_remove() is jsut a wrapper to sdhci_pltfm_unregister.
So use the sdhci_pltfm_unregister() for the .remove hook directly.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>