pwm: mediatek: Prevent divide-by-zero in pwm_mediatek_config()

With CONFIG_COMPILE_TEST && !CONFIG_HAVE_CLK, pwm_mediatek_config() has a
divide-by-zero in the following line:

	do_div(resolution, clk_get_rate(pc->clk_pwms[pwm->hwpwm]));

due to the fact that the !CONFIG_HAVE_CLK version of clk_get_rate()
returns zero.

This is presumably just a theoretical problem: COMPILE_TEST overrides
the dependency on RALINK which would select COMMON_CLK.  Regardless it's
a good idea to check for the error explicitly to avoid divide-by-zero.

Fixes the following warning:

  drivers/pwm/pwm-mediatek.o: warning: objtool: .text: unexpected end of section

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/fb56444939325cc173e752ba199abd7aeae3bf12.1742852847.git.jpoimboe@kernel.org
[ukleinek: s/CONFIG_CLK/CONFIG_HAVE_CLK/]
Fixes: caf065f ("pwm: Add MediaTek PWM support")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/9e78a0796acba3435553ed7db1c7965dcffa6215.1743501688.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>

drivers/pwm/pwm-mediatek.c

@@ -121,21 +121,25 @@ static int pwm_mediatek_config(struct pwm_chip *chip, struct pwm_device *pwm,
 	struct pwm_mediatek_chip *pc = to_pwm_mediatek_chip(chip);
 	u32 clkdiv = 0, cnt_period, cnt_duty, reg_width = PWMDWIDTH,
 	    reg_thres = PWMTHRES;
+	unsigned long clk_rate;
 	u64 resolution;
 	int ret;
 
 	ret = pwm_mediatek_clk_enable(chip, pwm);
-
 	if (ret < 0)
 		return ret;
 
+	clk_rate = clk_get_rate(pc->clk_pwms[pwm->hwpwm]);
+	if (!clk_rate)
+		return -EINVAL;
+
 	/* Make sure we use the bus clock and not the 26MHz clock */
 	if (pc->soc->has_ck_26m_sel)
 		writel(0, pc->regs + PWM_CK_26M_SEL);
 
 	/* Using resolution in picosecond gets accuracy higher */
 	resolution = (u64)NSEC_PER_SEC * 1000;
-	do_div(resolution, clk_get_rate(pc->clk_pwms[pwm->hwpwm]));
+	do_div(resolution, clk_rate);
 
 	cnt_period = DIV_ROUND_CLOSEST_ULL((u64)period_ns * 1000, resolution);
 	while (cnt_period > 8191) {