net: socionext: remove mmio reads on Tx

Currently the driver issues 2 mmio reads to figure out the number of
transmitted packets and clean them. We can get rid of the expensive
reads since BIT 31 of the Tx descriptor can be used for that.
We can also remove the budget counting of Tx completions since all of
the descriptors are not deliberately processed.

Performance numbers using pktgen are:
size  pre-patch(pps)  post-patch(pps)
64       362483           427916
128      358315           411686
256      352725           389683
512      215675           216464
1024     113812           114442

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>