broadband-bearer: fix cid check in disconnect_3gpp

A value 0 is used to denote an invalid/uninitialized CID. This patch
fixes a CID check in disconnect_3gpp() of MMBroadbandBearer such that it
disables all PDP contexts via AT+CGACT=0 when no specific CID is used
(i.e. cid == 0).

src/mm-broadband-bearer.c

@@ -1508,7 +1508,7 @@ disconnect_3gpp (MMBroadbandBearer *self,
                                            user_data);
 
     /* If no specific CID was used, disable all PDP contexts */
-    ctx->cgact_command = (cid >= 0 ?
+    ctx->cgact_command = (cid > 0 ?
                           g_strdup_printf ("+CGACT=0,%d", cid) :
                           g_strdup_printf ("+CGACT=0"));