serial: uart_async_rx: Avoid possible division by zero

Ensures that config->but_cnt is not zero.

Fixes #66800
CID #338107

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>
(cherry picked from commit 5822267)

drivers/serial/uart_async_rx.c

@@ -137,6 +137,7 @@ void uart_async_rx_reset(struct uart_async_rx *rx_data)
 int uart_async_rx_init(struct uart_async_rx *rx_data,
 		       const struct uart_async_rx_config *config)
 {
+	__ASSERT_NO_MSG(config->buf_cnt > 0);
 	__ASSERT_NO_MSG(config->length / config->buf_cnt <= UINT8_MAX);
 	memset(rx_data, 0, sizeof(*rx_data));
 	rx_data->config = config;