cdc-ether: usbnet_cdc_zte_status() can be static

Fixes the following sparse warning:

drivers/net/usb/cdc_ether.c:469:6: warning:
 symbol 'usbnet_cdc_zte_status' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>