stmmac: make stmmac_probe_config_dt return the platform data struct

Since stmmac_probe_config_dt() allocates the platform data structure
it is cleaner if it just returned this structure directly. This
function will later be used in the probe function in dwmac-* drivers.

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