regulator: st-pwm: Convert to get_voltage_sel

Also remove test for selector in st_pwm_regulator_set_voltage_sel, the checking
is already done in .list_voltage.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>