tty: max310x: Add rx-during-tx rs485 flag support

The driver currently sets the echo suppression bit by default when rs485
is enabled. Naturally it disables any data retrieval in rs485 mode while
RTSn is pushed up. The receiver gate (RX_) can be enabled just by clearing
(or not setting) the EchoSuprs bit of mode2 register. So by setting or
clearing the bit we implement the SER_RS485_RX_DURING_TX rs485 flag
support.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>