drivers: net: xgene: Implement the backward compatibility with the old and new firmwa...

drivers: net: xgene: Implement the backward compatibility with the old and new firmware w.r.t Tx completion IRQ interrupt.

This patch implements the backward compatibility with the old firmware where
the Tx completion IRQ interrupt was absent whereas incase of new firmware
the Tx completion IRQ interrupt is present.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: Suman Tripathi <stripathi@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>