Bluetooth: host: Fix resume advertiser race condition

This patch fixes an issue with the advertiser not resuming connectable
advertiser if the last bt_conn_unref happens from something other
than the disconnected callback.
In this case this last bt_conn_unref was from gatt.c notify_cb called
from application main thread.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>