zd1211rw: initial IBSS support

this adds initial IBSS support for the zydas zd1211rw:
convince driver that it can do IBSS mode. add mac80211 beacon_update callback.

IBSS merge and TSF updates don't work yet, but it makes the driver usable in
ad-hoc networks.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>