net: stmmac: dwmac-rk: use devm_stmmac_probe_config_dt()

Simplify the driver's probe() function by using the devres
variant of stmmac_probe_config_dt().

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>