pwm: stm32: Fix build warning with CONFIG_DMA_ENGINE disabled

Without dmaengine support, we get a harmless warning about an unused
function:

	drivers/pwm/pwm-stm32.c:166:12: error: 'stm32_pwm_capture' defined but not used [-Werror=unused-function]

Changing the #ifdef to an IS_ENABLED() check shuts up that warning and
is slightly nicer to read.

Fixes: 53e38fe7 ("pwm: stm32: Add capture support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-stm32.c

@@ -484,9 +484,7 @@ static int stm32_pwm_apply_locked(struct pwm_chip *chip, struct pwm_device *pwm,
 static const struct pwm_ops stm32pwm_ops = {
 	.owner = THIS_MODULE,
 	.apply = stm32_pwm_apply_locked,
-#if IS_ENABLED(CONFIG_DMA_ENGINE)
-	.capture = stm32_pwm_capture,
-#endif
+	.capture = IS_ENABLED(CONFIG_DMA_ENGINE) ? stm32_pwm_capture : NULL,
 };
 
 static int stm32_pwm_set_breakinput(struct stm32_pwm *priv,