net: stmmac: update rx tail pointer register to fix rx dma hang issue.

Currently we will not update the receive descriptor tail pointer in
stmmac_rx_refill. Rx dma will think no available descriptors and stop
once received packets exceed DMA_RX_SIZE, so that the rx only test will fail.

Update the receive tail pointer in stmmac_rx_refill to add more descriptors
to the rx channel, so packets can be received continually

Fixes: 54139cf3 ("net: stmmac: adding multiple buffers for rx")
Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>