mac80211: dont add BSS when creating IBSS

There's no need to create a BSS struct only to pass it to
ieee80211_sta_join_ibss, so refactor this function into
__ieee80211_sta_join_ibss which takes all the relevant
paramters, and ieee80211_sta_join_ibss which takes a BSS
struct (used when joining an IBSS that already has other
members).

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>