pwm: sun4i: Propagate errors in .get_state() to the caller

.get_state() can return an error indication now. Make use of it to
propagate an impossible prescaler encoding, should that have sneaked in
somehow.
Also check the return value of clk_get_rate(). That's unlikely to fail,
but we use that in two divide operations down in the code, so let's
avoid a divide-by-zero condition on the way.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20221201152223.3133-1-andre.przywara@arm.com
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-sun4i.c

@@ -118,6 +118,8 @@ static int sun4i_pwm_get_state(struct pwm_chip *chip,
 	unsigned int prescaler;
 
 	clk_rate = clk_get_rate(sun4i_pwm->clk);
+	if (!clk_rate)
+		return -EINVAL;
 
 	val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
 
@@ -142,7 +144,7 @@ static int sun4i_pwm_get_state(struct pwm_chip *chip,
 		prescaler = prescaler_table[PWM_REG_PRESCAL(val, pwm->hwpwm)];
 
 	if (prescaler == 0)
-		return 0;
+		return -EINVAL;
 
 	if (val & BIT_CH(PWM_ACT_STATE, pwm->hwpwm))
 		state->polarity = PWM_POLARITY_NORMAL;