Merge branch 'bugfix/uart_cannot_write_completely_3.1' into 'release/v3.1'

fix: fix uart write data incomplete(backport v3.1)

See merge request sdk/ESP8266_RTOS_SDK!1378

Author: donghengqaz

components/esp8266/driver/uart.c

@@ -562,7 +562,11 @@ static void uart_rx_intr_handler_default(void *param)
                         }
 
                         if (p_uart->tx_len_tot == 0) {
-                            en_tx_flg = false;
+                            if (tx_fifo_rem == 0) {
+                                en_tx_flg = true;
+                            } else{
+                                en_tx_flg = false;
+                            }
                             xSemaphoreGiveFromISR(p_uart->tx_done_sem, &task_woken);
                             if (task_woken == pdTRUE) {
                                 portYIELD_FROM_ISR();