serial: stm32: Deassert Transmit Enable on ->rs485_config()

[ Upstream commit adafbbf6 ]

The STM32 USART can control RS-485 Transmit Enable in hardware.  Since
commit 7df5081c ("serial: stm32: Add RS485 RTS GPIO control"),
it can alternatively be controlled in software.  That was done to allow
RS-485 even if the RTS pin is unavailable because it's pinmuxed to a
different function.

However the commit neglected to deassert Transmit Enable upon invocation
of the ->rs485_config() callback.  Fix it.

Avoid forward declarations by moving stm32_usart_tx_empty(),
stm32_usart_rs485_rts_enable() and stm32_usart_rs485_rts_disable()
further up in the driver.

Fixes: 7df5081c

 ("serial: stm32: Add RS485 RTS GPIO control")
Cc: stable@vger.kernel.org # v5.9+
Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/6059eab35dba394468335ef640df8b0050fd9dbd.1662886616.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>