net: ethernet: mtk_eth_soc: cache HW pointer of last freed TX descriptor

The value is only updated by the CPU, so it is cheaper to access from the
ring data structure than from a hardware register.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>