phylink: make local function phylink_phy_change() static

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