serial: 8250_fintek: Return -EINVAL on invalid configuration

Hardware does not support having the same RTS level during RX and TX
when RS485 mode is on (URA in Fintek terminology).

The manufacturer has also confirmed that the delays are not enabled if
the RS485 mode is not enabled.

Therefore we should return -EINVAL if the user wants to have the same
value for RTS_ON_SEND and RTS_AFTER_SEND.

Cc: "Ji-Ze Hong (Peter Hong)" <hpeter@gmail.com>
Cc: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>