dpaa_eth: fix SG frame cleanup

Fix issue with the entry indexing in the sg frame cleanup code being
off-by-1. This problem showed up when doing some basic iperf tests and
manifested in traffic coming to a halt.

Signed-off-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Acked-by: Madalin Bucur <madalin.bucur@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>