drivers: net: xgene: Correct probe sequence handling

The phy is connected at early stage of probe but not properly
disconnected if error occurs.  This patch fixes the issue.

Also changing the return type of xgene_enet_check_phy_handle(),
since this function always returns success.

Signed-off-by: Quan Nguyen <qnguyen@apm.com>
Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>