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-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)
 {