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 with GMAC5 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>