Fixes the following sparse warnings:

drivers/net/phy/phylink.c:570:6: warning:
 symbol 'phylink_phy_change' was not declared. Should it be static?
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>