net: stmmac: Fix case when PHY handle is not present

Some DT bindings do not have the PHY handle. Let's fallback to manually
discovery in case phylink_of_phy_connect() fails.

Changes from v1:
	- Fixup comment style (Sergei)

Fixes: 74371272 ("net: stmmac: Convert to phylink and remove phylib logic")
Reported-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>
Tested-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>
Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>