port-serial-qcdm: fix warnings with -Wsign-compare

mm-port-serial-qcdm.c: In function ‘find_qcdm_start’:
  mm-port-serial-qcdm.c:51:19: error: comparison of integer expressions of different signedness: ‘int’ and ‘guint’ {aka ‘unsigned int’} [-Werror=sign-compare]
     51 |     for (i = 0; i < response->len; i++) {
        |                   ^

src/mm-port-serial-qcdm.c

@@ -40,7 +40,8 @@ struct _MMPortSerialQcdmPrivate {
 static gboolean
 find_qcdm_start (GByteArray *response, gsize *start)
 {
-    int i, last = -1;
+    guint i;
+    gint  last = -1;
 
     /* Look for 3 bytes and a QCDM frame marker, ie enough data for a valid
      * frame.  There will usually be three cases here; (1) a QCDM frame
@@ -50,7 +51,7 @@ find_qcdm_start (GByteArray *response, gsize *start)
      */
     for (i = 0; i < response->len; i++) {
         if (response->data[i] == 0x7E) {
-            if (i > last + 3) {
+            if (i > (guint)(last + 3)) {
                 /* Got a full QCDM frame; 3 non-0x7E bytes and a terminator */
                 if (start)
                     *start = last + 1;