net: stmmac: fix double-initialization of phy_iface

The variable phy_iface is double-initialized to itself.
This patch remove that.

Reported-by: coverity (CID 1271141)
Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>