When a BSSID is joined, set the link status to 'preassoc', and set it to
'idle' when the BSS is deleted.

This is what the downstream driver is doing, and it seems to improve the
reliability during connect/disconnect stress tests.
Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>