Revert "mmc: sdhci-iproc: Fix vmmc regulators on iProc"

This reverts commit aed19399.

Commit 6c92ae1e ("mmc: sdhci: Introduce sdhci_set_power_and_bus_voltage()")
introduced a generic helper that does the same thing so use that instead in
the following commit.

Signed-off-by: Juerg Haefliger <juergh@canonical.com>