serial: 8250: Handle UART without interrupt on TEMT using em485

Introduce the UART_CAP_NOTEMT capability. The capability indicates that
the UART doesn't have an interrupt available on TEMT.

In the case where the device does not support it, we calculate the
maximum time it could take for the transmitter to empty the
shift register. When we get in the situation where we get the
THRE interrupt, we check if the TEMT bit is set. If it's not, we start
the a timer and recall __stop_tx() after the delay.

The transmit sequence is a bit modified when the capability is set. The
new timer is used between the last interrupt(THRE) and a potential
stop_tx timer.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
[moved to use added UART_CAP_TEMT]
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
[moved to use added UART_CAP_NOTEMT, improve timeout]
Signed-off-by: Eric Tremblay <etremblay@distech-controls.com>
[rebased to v5.17, making use of tty_get_frame_size]
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20220330104642.229507-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>