mmc: mmci: Switch to mmc_regulator_set_vqmmc()

mmc: mmci: Switch to mmc_regulator_set_vqmmc()

Instead of reimplementing the logic in mmc_regulator_set_vqmmc(), use the
mmc code function directly.

This also allows us to fix a related issue on STM32MP1, when a voltage
switch of 1.8V is done for the eMMC, but the current level is already set
to 1.8V. More precisely, in this scenario the call to the
->post_sig_volt_switch() hangs, indefinitely waiting for the voltage switch
to complete. Fix this problem by checking if mmc_regulator_set_vqmmc()
returned 1 and then skip invoking the callback.

Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20200416163649.336967-3-marex@denx.de
[Ulf: Updated the commit message]
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>