app/testpmd: revert MAC update in checksum forwarding

app/testpmd: revert MAC update in checksum forwarding

This patch reverts
commit 10f4620f02e1 ("app/testpmd: modify mac in csum forwarding"),
as the checksum forwarding is expected to only perform
checksum and not also overwrites the source and destination MAC addresses.

Doing so, we can test checksum offloading with real traffic
without breaking broadcast packets.

Fixes: 10f4620f02e1 ("app/testpmd: modify mac in csum forwarding")
Cc: stable@dpdk.org
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Chenbo Xia <chenbo.xia@intel.com>
Acked-by: Aman Singh <aman.deep.singh@intel.com>