pcmcia serial_cs.c: fix multifunction card handling

We shouldn't overwrite pre-set values, and we should also
set the port address to the beginning, and not the end of
the 8-port range.

CC: linux-serial@vger.kernel.org
Reported-by: Komuro <komurojun-mbn@nifty.com>
Hardware-supplied-by: Jochen Frieling <j.frieling@pengutronix.de>
Tested-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>