dpaa2-eth: Fix minor bug in ethtool stats reporting

Don't print error message for a successful return value.

Fixes: d84c3a4d

 ("dpaa2-eth: Add new DPNI statistics counters")

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>