sierra: fix detection of APP1 port

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

commit e0242b4db7fb1556e79f6829d22edf411f9f6ba4
Author: Dan Williams <dcbw@redhat.com>
Date:   Thu Aug 23 21:14:38 2012 -0500

    sierra: fix detection of APP1 port

    The APP1 port (which has a limited AT parser) doesn't prefix
    its replies with <CR><LF> like nice modems do, and that means
    it runs afoul of the echo removal bits of the AT serial port
    code.  We need to parse the whole string even though it's not
    prefixed properly to find the APP1 string in the response.

plugins/sierra/mm-plugin-sierra.c

@@ -261,6 +261,7 @@ mm_plugin_create (void)
                       MM_PLUGIN_ALLOWED_QCDM,       TRUE,
                       MM_PLUGIN_CUSTOM_INIT,        &custom_init,
                       MM_PLUGIN_ICERA_PROBE,        TRUE,
+                      MM_PLUGIN_REMOVE_ECHO,        FALSE,
                       NULL));
 }