net: stmmac: dwxgmac2: Also use TBU interrupt to clean TX path

TBU interrupt is a normal interrupt and can be used to trigger the
cleaning of TX path. Lets check if it's active in DMA interrupt handler.

While at it, refactor a little bit the function:
	- Don't check if RI is enabled because at function exit we will
	  only clear the interrupts that are enabled so, no event will
	  be missed.

In my tests withe XGMAC2 this increased performance.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>