8250/fintek: Use rs485 handler from serial_core

In order to remove the handler for rs485 ioctls on serial_8250, all the
drivers must use the implementation on serial_core.

Reviewed-by: Alan Cox <alan@linux.intel.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>