net/vmxnet3: Refactor 'vmxnet_rx_pkt_attach_data'

Separate RX packet protocol parsing out of 'vmxnet_rx_pkt_attach_data'.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@ravellosystems.com>
Message-id: 1436864116-19154-2-git-send-email-shmulik.ladkani@ravellosystems.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>