broadband-modem: fix warnings with -Wsign-compare

mm-broadband-modem.c: In function ‘setup_ports’:
  mm-broadband-modem.c:9955:23: error: comparison of integer expressions of different signedness: ‘gint’ {aka ‘int’} and ‘guint’ {aka ‘unsigned int’} [-Werror=sign-compare]
   9955 |         for (j = 0; j < array->len; j++) {
        |                       ^
  ...

src/mm-broadband-modem.c

@@ -10000,7 +10000,7 @@ setup_ports (MMBroadbandModem *self)
     MMPortSerialAt *ports[2];
     GRegex *regex;
     GPtrArray *array;
-    gint i, j;
+    guint i, j;
 
     ports[0] = mm_base_modem_peek_port_primary (MM_BASE_MODEM (self));
     ports[1] = mm_base_modem_peek_port_secondary (MM_BASE_MODEM (self));