net: ethernet: stmmac: platform: fix outdated function header

Fix the OF-helper function header to reflect that the function no longer
has a platform-data parameter.

Fixes: b0003ead ("stmmac: make stmmac_probe_config_dt return the
platform data struct")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>