zd1211rw: port to mac80211

This seems to be working smoothly now. Let's not hold back the mac80211
transition any further. This patch ports the existing driver from softmac
to mac80211.

Many thanks to everyone who helped out with the porting efforts.

Signed-off-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>