mac80211 ibss: flush only stations belonging to current interface

When joining a new IBSS, all old stations are flushed, but currently
all stations belonging to all virtual interfaces are flushed, which
is wrong. This patch fixes it.

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