mmc: sdhci: using common mmc_regulator_set_vqmmc()
Switch to use the more robust common mmc_regulator_set_vqmmc()
function in MMC core which set the target voltage as close as
possible to target voltage.
We did not re-factor the whole sdhci_start_signal_voltage_switch()
cause we want to keep the original signal switch order between host
and card to avoid potential break.