amd-xgbe: Separate Tx/Rx ring data fields into new structs

Move the Tx and Rx related fields within the xgbe_ring_data struct into
their own structs in order to more easily see what fields are used for
each operation.

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