mmc: mediatek: Use mmc_regulator_set_vqmmc in start_signal_voltage_switch

We've introduced a new helper in the MMC core:
mmc_regulator_set_vqmmc().  Let's use this in mtk-sd.  Using this new
helper has some advantages:

    1. We get the mmc_regulator_set_vqmmc() behavior of trying to match
       VQMMC and VMMC when the signal voltage is 3.3V.  This ensures max
       compatibility.

    2. We get rid of a few more warnings when probing unsupported
       voltages.

    3. We get rid of some non-mediatek specific code in mtk-sd.

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>