amd-xgbe: Check for Tx hardware queue flushing support

The flushing of the Tx hardware queues is only supported at a certain
level of the hardware.  Retrieve the current version of the hardware
and use that to determine if flushing is supported.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>