serial: stm32: Re-assert RTS/DE GPIO in RS485 mode only if more data are transmitted

serial: stm32: Re-assert RTS/DE GPIO in RS485 mode only if more data are transmitted

The stm32_usart_transmit_chars() may be called with empty or stopped
transmit queue, and no XON/OFF character pending. This can happen at
the end of transmission, where this last call is used to either handle
the XON/XOFF x_char, or disable TX interrupt if queue is empty or
stopped.

If that occurs, do not assert the RS485 RTS/DE GPIO anymore, as the
GPIO would remain asserted past the end of transmission and that would
block the RS485 bus after the transmission.

Only assert the RS485 RTS/DE GPIO if there is either pending XON/XOFF
x_char, or at least one character in running transmit queue.

Fixes: d7c76716169d ("serial: stm32: Use TC interrupt to deassert GPIO RTS in RS485 mode")
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20230223042252.95480-2-marex@denx.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>