serial: max310x: Add support for RS-485 mode

This patch adds support for RS-485 (TIOCSRS485/TIOCGRS485) IOCTLs.
As a result this patch eliminate private driver header.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>