amd-xgbe: Remove need for Tx path spinlock

Since the Tx ring cleanup can run at the same time that data is being
transmitted, a spin lock was used to protect the ring. This patch
eliminates the need for Tx spinlocks by updating the current ring
position only after all ownership bits for data being transmitted have
been set. This will insure that ring operations in the Tx cleanup path
do not interfere with the ring operations in the Tx transmit path.

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