amd-xgbe: Simplify the Rx desciptor ring tracking

Make the Rx descriptor ring processing similar to the Tx descriptor
ring processing.  Remove the realloc_index and realloc_threshold
variables and base everything on the current index counter and the
dirty index counter.

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