serial: stm32: Factor out GPIO RTS toggling into separate function

Pull out the GPIO RTS enable and disable handling into separate function.
Limit the scope of GPIO RTS toggling only to GPIO emulated RS485 too.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Erwan Le Ray <erwan.leray@foss.st.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jean Philippe Romain <jean-philippe.romain@foss.st.com>
Cc: Valentin Caron <valentin.caron@foss.st.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-serial@vger.kernel.org
Link: https://lore.kernel.org/r/20220430162845.244655-1-marex@denx.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>