mac80211: Adds clean sdata helper

Adds hepler to clean sdata ieee80211_clean_sdata similar way as
ieee80211_setup_sdata is implemented. The function will be used by other
interfaces later.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>