net: stmmac: fix INTR TBU status affecting irq count statistic

DMA channel status "Transmit buffer unavailable(TBU)" bit is not
considered as a successful dma tx. Hence, it should not affect
all the irq count statistic.

Fixes: 1103d3a5 ("net: stmmac: dwmac4: Also use TBU interrupt to clean TX path")
Signed-off-by: Voon Weifeng <weifeng.voon@intel.com>
Signed-off-by: Vijayakannan Ayyathurai <vijayakannan.ayyathurai@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>