net: dsa: tag_ocelot_8021q: calculate TX checksum in software for deferred packets

DSA inherits NETIF_F_CSUM_MASK from master->vlan_features, and the
expectation is that TX checksumming is offloaded and not done in
software.

Normally the DSA master takes care of this, but packets handled by
ocelot_defer_xmit() are a very special exception, because they are
actually injected into the switch through register-based MMIO. So the
DSA master is not involved at all for these packets => no one calculates
the checksum.

This allows PTP over UDP to work using the ocelot-8021q tagging
protocol.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>