ims_charging: fix wrong default User-Equipment-Info-Type AVP format

- fix User-Equipment-Info-Type AVP format
  According to the RFC 8506 it should be like this: The 48-bit Media Access
  Control (MAC) address is formatted as described in Section 3.21 of [RFC3580]
- use strlen instead of sizeof

src/modules/ims_charging/ims_ro.c

@@ -1615,9 +1615,8 @@ static int get_mac_avp_value(struct sip_msg *msg, str *value) {
 
     pv_parse_spec2(&mac_avp_name_str, &avp_spec, 1);
     if (pv_get_spec_value(msg, &avp_spec, &val) != 0 || val.rs.len == 0) {
-
-        value->s = "00:00:00:00:00:00";
-        value->len = sizeof ("00:00:00:00:00:00") - 1;
+        value->s = "00-00-00-00-00-00";
+        value->len = strlen(value->s);
         return -1;
     }