net: stmmac: reduce indentation by adding a continue

As suggested by Joe Perches, replacing the "if phydev" logic permit to
reduce indentation in the for loop.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Reviewed-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>