net: stmmac: introduce DMA interrupt status masking per traffic direction

In preparation to make stmmac support multi-vector MSI, we introduce the
interrupt status masking according to RX, TX or RXTX. Default to use RXTX
inside stmmac_dma_interrupt(), so there is no run-time logic difference
now.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: Voon Weifeng <weifeng.voon@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>