cxgb4: Add support for FW_ETH_TX_PKT_VM_WR

The present TX workrequest(FW_ETH_TX_PKT_WR) cant be used for
host->vf communication, since it doesn't loopback the outgoing
packets to virtual interfaces on the same port. This can be done
using FW_ETH_TX_PKT_VM_WR.
This fix depends on ethtool_flags to determine what WR to use for
TX path. Support for setting this flags by user is added in next
commit.

Based on the original work by : Casey Leedom <leedom@chelsio.com>

Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: Arjun Vynipadath <arjun@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>