net: stmmac: dwmac-intel-plat: remove unnecessary initialization

plat_dat is initialized by stmmac_probe_config_dt().
So, initialization is not required by priv->plat.
This removes unnecessary initialization and variables.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>