amd-xgbe: Prevent Tx cleanup stall

When performing Tx cleanup, the dirty index counter is compared to the
current index counter as one of the tests used to determine when to stop
cleanup. The "less than" test will fail when the current index counter
rolls over to zero causing cleanup to never occur again. Update the test
to a "not equal" to avoid this situation.

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