sierra: skip echo removal in the secondary port

This is the port to git master of the following commit:

commit d1be19d231a395339b1f452d1a30b73ea77ad528
Author: Dan Williams <dcbw@redhat.com>
Date:   Tue Aug 28 21:58:43 2012 -0500

    sierra: fix CSQ handling on APP1 port

    The APP1 port doesn't always prefix its replies with <CR><LF> which
    runs afoul of the built-in echo removal.  Since Sierra modems are on
    the whole well-behaved WRT echo removal, just disable it on the
    secondary ports.  Only changes behavior for PPP-based devices since
    they are the only ones that use the APP1 ports.

plugins/sierra/mm-common-sierra.c

@@ -215,6 +215,15 @@ mm_common_sierra_setup_ports (MMBroadbandModem *self)
                       MM_PORT_CARRIER_DETECT, FALSE,
                       NULL);
 
+        if (i == 1) {
+            /* Built-in echo removal conflicts with the APP1 port's limited AT
+             * parser, which doesn't always prefix responses with <CR><LF>.
+             */
+            g_object_set (ports[i],
+                          MM_AT_SERIAL_PORT_REMOVE_ECHO, FALSE,
+                          NULL);
+        }
+
         mm_at_serial_port_add_unsolicited_msg_handler (
             ports[i],
             pacsp0_regex,