broadband-modem: skip +CGMM: prefix when loading device model

Some devices, e.g. ZTE MF820D, seem to prefix the `AT+CGMM?' response with the
`+CGMM:' string, resulting in the following model string being loaded:

   model: '+CGMM: "MF820D"'

Avoid this by:
 1) Removing the expected prefixes.
 2) Unquoting the resulting string.

Reported by: Marius Kotsbak <marius.kotsbak@gmail.com>

src/mm-broadband-modem.c

@@ -634,13 +634,23 @@ modem_load_model_finish (MMIfaceModem *self,
                          GError **error)
 {
     GVariant *result;
+    const gchar *p;
     gchar *model;
 
     result = mm_base_modem_at_sequence_finish (MM_BASE_MODEM (self), res, NULL, error);
     if (!result)
         return NULL;
 
-    model = g_strstrip (g_variant_dup_string (result, NULL));
+    p = g_variant_get_string (result, NULL);
+
+    /* Some devices (e.g. ZTE MF820D) seem to include the command prefix */
+    p = mm_strip_tag (p, "+CGMM:");
+    p = mm_strip_tag (p, "+GMM:");
+    model = g_strdup (p);
+
+    /* Stripping quotes modifies string in place */
+    model = mm_strip_quotes (model);
+
     mm_dbg ("loaded model: %s", model);
     return model;
 }

src/mm-modem-helpers.c

@@ -32,6 +32,23 @@
 
 /*****************************************************************************/
 
+gchar *
+mm_strip_quotes (gchar *str)
+{
+    gsize len;
+
+    if (!str)
+        return NULL;
+
+    len = strlen (str);
+    if ((len >= 2) && (str[0] == '"') && (str[len - 1] == '"')) {
+        str[0] = ' ';
+        str[len - 1] = ' ';
+    }
+
+    return g_strstrip (str);
+}
+
 const gchar *
 mm_strip_tag (const gchar *str, const gchar *cmd)
 {

src/mm-modem-helpers.h

@@ -44,6 +44,7 @@
     (MM_MODEM_CAPABILITY_GSM_UMTS |     \
      MM_MODEM_CAPABILITY_3GPP_LTE)
 
+gchar       *mm_strip_quotes (gchar *str);
 const gchar *mm_strip_tag    (const gchar *str,
                               const gchar *cmd);