mmc: core: Rename __mmc_set_signal_voltage() to mmc_set_signal_voltage()

Earlier the mmc_set_signal_voltage() existed, but since it has been renamed
to mmc_set_uhs_voltage(), we can now use that name instead.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Tested-by: Jan Glauber <jglauber@cavium.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>