[nrf fromlist] drivers: serial: fix cfg.baudrate may be used uninitialized

drivers/serial/uart_async_to_irq.c: In function 'get_rx_timeout':
drivers/serial/uart_async_to_irq.c:51:26:
warning: 'cfg.baudrate' may be used uninitialized
   51 |                 baudrate = cfg.baudrate;
      |                 ~~~~~~~~~^~~~~~~~~~~~~~
drivers/serial/uart_async_to_irq.c:45:28: note: 'cfg' declared here
   45 |         struct uart_config cfg;

Upstream PR: zephyrproject-rtos/zephyr#74530

Signed-off-by: Piotr Kosycarz <[email protected]>

Author: nordic-piks

drivers/serial/uart_async_to_irq.c

@@ -42,7 +42,7 @@ static const struct uart_async_to_irq_config *get_config(const struct device *de
 /* Function calculates RX timeout based on baudrate. */
 static uint32_t get_rx_timeout(const struct device *dev)
 {
-	struct uart_config cfg;
+	struct uart_config cfg = { 0 };
 	int err;
 	uint32_t baudrate;