pwm: stm32: Refuse too small period requests

If period_ns is small, prd might well become 0. Catch that case because
otherwise with

	regmap_write(priv->regmap, TIM_ARR, prd - 1);

a few lines down quite a big period is configured.

Fixes: 7edf7369 ("pwm: Add driver for STM32 plaftorm")
Cc: stable@vger.kernel.org
Reviewed-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/b86f62f099983646f97eeb6bfc0117bb2d0c340d.1718979150.git.u.kleine-koenig@baylibre.comSigned-off-by: Uwe Kleine-König <ukleinek@kernel.org>

drivers/pwm/pwm-stm32.c

@@ -337,6 +337,8 @@ static int stm32_pwm_config(struct stm32_pwm *priv, unsigned int ch,
 
 	prd = mul_u64_u64_div_u64(period_ns, clk_get_rate(priv->clk),
 				  (u64)NSEC_PER_SEC * (prescaler + 1));
+	if (!prd)
+		return -EINVAL;
 
 	/*
 	 * All channels share the same prescaler and counter so when two