net: stmmac: Populate missing callbacks in HWIF initialization

Some HW specific setups, like sun8i, do not populate all the necessary
callbacks, which is what HWIF helpers were expecting.

Fix this by always trying to get the generic helpers and populate them
if they were not previously populated by HW specific setup.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Fixes: 5f0456b4 ("net: stmmac: Implement logic to automatically
select HW Interface")
Reported-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Cc: Corentin Labbe <clabbe.montjoie@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>