regulator: wm8994: Convert to set_voltage_sel()

There's no need to implement set_voltage() as there is only a very small
range of selectors for the regulators in the WM8994 and the voltages are
not expected to vary frequently at runtime.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

diff --cc drivers/regulator/wm8994-regulator.c
Simple merge