net: systemport: Turn on offloads by default

We can turn on the RX/TX checksum offloads by default and make sure that
those are properly reflected back to e.g: stacked devices such as VLAN
or DSA.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>