stmmac: drop unnecessary dt checks in stmmac_probe_config_dt

Since the caller already check the presence of a of_node there
is no need to repeat the check in stmmac_probe_config_dt.

There is also no point in checking the return value of the
of_match_device function since if there wasn't match in the
first place we would never be in this function.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>