brcmsmac: implement ieee80211_ops get_tsf and set_tsf

setting the tsf is needed to start a timer to make beaconing in AP mode
work.

This is based on older versions of brcmsmac and b43.

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