brcmsmac: add beacon template support

This makes it possible that a beacon template provided by mac80211 is
written to the hardware for constant beaconing.

This is based on an old version of brcmsmac, on b43 and the spec b43 is
based on.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>