wlcore: fixes for connection_loss_work

We can't use cancel_delayed_work_sync() from functions that take the
wl->mutex, since connection_loss_work also takes the mutex. This might
result in a deadlock. Restructure the code so the work is synchronously
canceled before taking the mutex.
Avoid a bug where we would indefinitely delay the connection loss
indication by re-queuing the connection loss work on consecutive beacon
loss events.

Cc: bartosz.markowski <bartosz.markowski@tieto.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>