AA: mac80211: b43: load b43 on core rev 17 and 18.

This is now controlled by a module parameter and the default 
configuration depends on brcmsmac being installed.
This patch causes b43 to take core rev 17 and 18 always by default.

Backport of r37307


git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@37308 3c298f89-4303-0410-b956-a3cf2f4a3e73