net/enic: add Tx prepare handler

net/enic: add Tx prepare handler

Like most NICs, this hardware (Cisco VIC) also requires partial
checksum in the packet for checksum offload and TSO. So, add
the tx_pkt_prepare handler like other PMDs do.

Technically, VIC has an offload mode that does not require partial
checksum for non-TSO packets. But, it has no such mode for TSO
packets, making tx_pkt_prepare unavoidable.

Signed-off-by: Hyong Youb Kim <hyonkim@cisco.com>
Reviewed-by: John Daley <johndale@cisco.com>