fix(demo): fix missing ecm mac address string

Signed-off-by: sakumisu <1203593632@qq.com>

demo/cdc_ecm_template.c

@@ -68,6 +68,7 @@ static const char *string_descriptors[] = {
     "CherryUSB",                  /* Manufacturer */
     "CherryUSB CDC ECM DEMO",     /* Product */
     "2022123456",                 /* Serial Number */
+    "aabbccddeeff",               /* ecm mac address */
 };
 
 static const uint8_t *device_descriptor_callback(uint8_t speed)