Always arm the "TX FIFO Empty" interrupt after we write into _tx_buffer.

This avoids a race where the interrupt handler detects an empty _tx_buffer
just before we write data into it.

Note that commit d6f6294 works around
this race when data is continually added to _tx_buffer in the hung state.
We revert that change here as the race should no longer occur.

Testing performed:
 - set UART_CONF1.txfifo_empty_thrhd=0x70 (which exacerbates the issue)
 - generate a ~240 byte burst of data, sent in back-to-back Serial1.write(, 4)
   calls, optionally followed by a Serial1.flush()
Test results:
 - before this change, observe occasional unsent data and hang in flush()
   (if used).
 - after this change, data is sent as expected.

Author: alltheblinkythings

cores/esp8266/HardwareSerial.cpp

@@ -617,18 +617,15 @@ size_t HardwareSerial::write(uint8_t c) {
     size_t room = uart_get_tx_fifo_room(_uart);
     if(room > 0 && _tx_buffer->empty()) {
         uart_transmit_char(_uart, c);
-        if(room < 10) {
-            uart_arm_tx_interrupt(_uart);
-        }
         return 1;
     }
 
     while(_tx_buffer->room() == 0) {
         yield();
-        uart_arm_tx_interrupt(_uart);
     }
 
     _tx_buffer->write(c);
+    uart_arm_tx_interrupt(_uart);
     return 1;
 }