stmmac: mdio register has to fail if the phy is not found

With this patch the stmmac fails in case of the phy device
is not found; w/o this fix the mdio can be register twice when
do down/up the iface and this is not correct.

Reported-by: Stas <stsp@list.ru>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>