net: ethernet: mtk-star-emac: separate tx/rx handling with two NAPIs

Current driver may lost tx interrupts under bidirectional test with iperf3,
which leads to some unexpected issues.

This patch let rx/tx interrupt enable/disable separately, and rx/tx are
handled in different NAPIs.

Signed-off-by: Biao Huang <[email protected]>
Signed-off-by: Yinghua Pan <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: Biao Huang