Fix DTR offset value (esp8266#8586)

Correct value from uart_registers.h mask
> #define UART_DTRN (BIT(29))

cores/esp8266/esp8266_peri.h

@@ -255,7 +255,7 @@ extern volatile uint32_t* const esp8266_gpioToFn[16];
 //UART STATUS Registers Bits
 #define USTX    31 //TX PIN Level (Doesn't seem to work, always reads as 0 for both uarts. HW bug? Possible workaround: Enable loopback UxC0 |= 1<<UCLBE and read USRXD, see https://github.com/esp8266/Arduino/issues/7256 for discussion.)
 #define USRTS   30 //RTS PIN Level
-#define USDTR   39 //DTR PIN Level
+#define USDTR   29 //DTR PIN Level
 #define USTXC   16 //TX FIFO COUNT (8bit)
 #define USRXD   15 //RX PIN Level
 #define USCTS   14 //CTS PIN Level