mac80211: fix ibss join with fixed-bssid

When fixed bssid is requested when joining an ibss network, incoming
beacons that match the configured bssid cause mac80211 to create new
sta entries, even before the ibss interface is in joined state.
When that happens, it fails to bring up the interface entirely, because
it checks for existing sta entries before joining.
This patch fixes this bug by refusing to create sta info entries before
the interface is fully operational.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>