The mvneta device requires MVNETA_TX_CMD_L4_CHECKSUM_NONE bit to be set in
the tx descriptor is checksum not required. However, mvneta_tx_set_csumflag()
is not setting this flag currently, causing the hardware to randomly corrupt IP
header during transmission.
This affects injected IPv4 packets that skips kernel IP stack processing
(e.g. DHCP), as well as all IPv6 packets, since the driver currently does not
offload csum for IPv6.
The fix is to remove all the early return paths from mvneta_tx_set_csumflag()
which do not set the MVNETA_TX_CMD_L4_CHECKSUM_NONE flag.