serial: stm32: update PIO transmission

Improves PIO transmission:
- Replaces the FIFO filling per character by a filling per blocks of
  characters, which provides better performances
- Replaces the active waiting loop by TX empty interrupt dynamic handling.
  TXE interrupt is now enabled when data has to be sent (ie when
  uart_circ is not empty), and inhibited when there is no more data to
  send (ie when uart_circ is empty).

Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>