mac80211: fix sta-info pinning

When a STA is supposed to be unlinked but is pinned, it still needs
to be unlinked from all structures. Only at the end of the unlink
process should we check for pin status and invalidate the callers
reference if it is pinned. Move the pin status check down.

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