regulator: Rework s5m8767_set_voltage to support both LDOs and BUCKs

s5m8767_set_voltage not only implement set_voltage callbacks for LDOs,
but also for BUCKs when s5m8767->buck_gpioindex is not set.
s5m8767_set_voltage_buck actually only for buck[2|3|4] when
s5m8767->buck_gpioindex is set.
Conditionally calling s5m8767_set_voltage in s5m8767_set_voltage_buck makes
the code hard to read.

I think merging s5m8767_set_voltage_buck and s5m8767_set_voltage actually
simplifies things. It's easy to use buck234_vol pointer to differentiate if
we need to control voltage for buck[2|3|4] by DVS GPIOs.

This patch reworks s5m8767_set_voltage to support both LDOx and BUCKx in all
cases.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Sangbeom Kim <sbkim73@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>