mvpp2: refactor the HW checksum setup

The hardware can only offload checksum calculation on first port due to
the Tx FIFO size limitation, and has a maximum L3 offset of 128 bytes.
Document this in a comment and move duplicated code in a function.

Fixes: 576193f2 ("net: mvpp2: jumbo frames support")
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>