regulator: pwm: Drop unneeded pwm_enable() call

Now that the PWM regulator driver implements the ->enable/disable() hooks
we can remove the pwm_enable() call from pwm_regulator_set_voltage().

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>