iface-modem: fix modem state consolidation upon bearer disconnection

Patch "iface-modem: fix invalid modem state consolidation" (commit
69aff6183a9e6532b4074c89831d6dcfa81ddcce) incorrectly consolidates the
modem state upon the disconnection of a bearer. The modem state remains
'connected' after the last bearer is disconnected. This patch fixes
that.
(cherry picked from commit bb73ce0aaf7c8ae4354d34412880a07f82b8a23e)
diff --git a/src/mm-iface-modem.c b/src/mm-iface-modem.c
index 8ae1e45..fd3b2b9 100644
--- a/src/mm-iface-modem.c
+++ b/src/mm-iface-modem.c
@@ -270,7 +270,7 @@
             new_state = MM_MODEM_STATE_DISCONNECTING;
             break;
         case MM_BEARER_STATUS_DISCONNECTED:
-            new_state = get_current_consolidated_state (self, state);
+            new_state = get_current_consolidated_state (self, MM_MODEM_STATE_UNKNOWN);
             break;
         }