net: phy: xgmiitorgmii: Fix non static symbol warning

Fixes the following sparse warning:

drivers/net/phy/xilinx_gmii2rgmii.c:61:5: warning:
 symbol 'xgmiitorgmii_probe' was not declared. Should it be static?

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