regulator: pwm: Fix handling of missing init voltage

Since priv->init_voltage is an unsigned integer it can never be
negative.  So the current code fails to detect a missing
'regulator-init-microvolt' property and instead misconfigures the
PWM device.  Fix this by making the relevant members of
'struct pwm_regulator_info' signed integers.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>