add Marvell 88W8688 support to libertas_sdio

This chip is for example used in the GuruPlug.
This patch avoids the following error:
libertas_sdio: failed to load firmware
libertas_sdio: probe of mmc0:0001:1 failed with error -5

The fix is based on code in:
drivers/net/wireless/libertas_uap/uap_sdio_mmc.c

This file can for example be found on the following links:
http://www.xilka.com/sheeva/2.6/2.6.36/2.6.36.2/source/0002-Driver-for-Marvell-Libertas-8688-SDIO-micro-AP-suppo-2.6.35.patch
http://www.downloadsnewit.co.uk/kernel-v3.0.7/

I followed the following wiki to setup a working WiFi client mode
connection on the GuruPlug:
http://wiki.debian.org/libertas



Signed-off-by: Harro Haan <hrhaan@gmail.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>