mac80211: disable ipw2x00 for big endian targets

The ipw2x00 drivers assume that the system they are running is little
endian, and access everything in native byte order. When run on a big
endian system, everything breaks apart.

Since fixing this is non trivial on a first glance, disable them for
big endian targets.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 46708