Thanks to Jussi Kivilinna

   https://bitbucket.org/nuttx/nuttx/commits/9169ff6a15fe65ba4af134b470dbf89220274c19

   stm32_serial: fix freezing serial port.  Serial interrupt enable/disable functions
   do not disable interrupts and can freeze device when serial interrupt is received
   while execution is at those functions.

   Trivially triggered with two or more threads write to regular syslog stream and to
   emergency stream. In this case, freeze happens because of mismatch of priv->ie
   (TXEIE == 0) and actually enabled interrupts in USART registers (TXEIE == 1),
   which leads to unhandled TXE interrupt and causes interrupt storm for USART.