coding guidelines: comply with MISRA Rule 20.9

- avoid to use undefined macros in #if expressions

Signed-off-by: Hess Nathan <nhess@baumer.com>

drivers/serial/uart_nrfx_uarte.c

@@ -548,7 +548,7 @@ static void tx_start(const struct device *dev, const uint8_t *buf, size_t len)
 	const struct uarte_nrfx_config *config = dev->config;
 	NRF_UARTE_Type *uarte = get_uarte_instance(dev);
 
-#if CONFIG_PM_DEVICE
+#ifdef CONFIG_PM_DEVICE
 	enum pm_device_state state;
 
 	(void)pm_device_state_get(dev, &state);