amd-xgbe: Add a read memory barrier to Tx/Rx path

Add a read memory barrier to the Tx and Rx paths where the ownership
bit is checked to be sure that all descriptor fields are read after
having read the ownership bit for the descriptor.

This has not been an issue to date, but it's a good safe-guard to have.

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