b43: fix ieee80211_rx() context

Due to the way it interacts with the networking
stack and other parts of mac80211, ieee80211_rx()
must be called with disabled softirqs.

[1] http://thread.gmane.org/gmane.linux.kernel.wireless.general/39440/focus=40266

Reported-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>