broadband-bearer: don't send disconnect CGACT on data port for multi-port modems

A Huawei E173 (with FW 11.126.15.00.445) seems to stop responding on the
primary port (ttyUSB2) when the CGACT is resent on the data port (ttyUSB0).
The CGACT-on-data-port was originally added as a fallback attempt to get
single-port modems to disconnect when all other methods failed.

For single-port modems, the primary port is also the data port so this
patch will have no effect.

For multi-port modems, this patch will retry the CGACT on the primary
port which has a higher chance of success because we already know
the primary port is talking to us, while the data port may still
be stuck in PPP mode.

src/mm-broadband-bearer.c

@@ -1511,10 +1511,18 @@ data_flash_3gpp_ready (MMPortSerial *data,
         return;
     }
 
-    /* Last resort, try to send CGACT in the data port itself */
-    mm_dbg ("Sending PDP context deactivation in data port...");
+    /* Send another CGACT on the primary port (also the data port when the modem
+     * only has one serial port) if the previous one failed.  Some modems, like
+     * the Huawei E173 (fw 11.126.15.00.445) stop responding on their primary
+     * port when the CGACT is sent on the separte data port.
+     */
+    if (MM_PORT_SERIAL (ctx->primary) == data)
+        mm_dbg ("Sending PDP context deactivation in primary/data port...");
+    else
+        mm_dbg ("Sending PDP context deactivation in primary port again...");
+
     mm_base_modem_at_command_full (ctx->modem,
-                                   MM_PORT_SERIAL_AT (data),
+                                   ctx->primary,
                                    ctx->cgact_command,
                                    10,
                                    FALSE,