net-next: stmmac: dwmac-mediatek: add module license info

Add MODULE_LICENSE info to fix this:
WARNING: modpost: missing MODULE_LICENSE() in drivers/net/ethernet/stmicro/stmmac/dwmac-mediatek.o

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>