regulator: Fix setting new voltage in s5m8767_set_voltage

Current code does not really update the register with new value, fix it.
I rename the variable i to sel for better readability.

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>