net/enic: add the simple version of Tx handler

net/enic: add the simple version of Tx handler

Add a much-simplified handler that works when all offloads are
disabled, except mbuf fast free. When compared against the default
handler, under ideal conditions, cycles per packet drop by 60+%.
The driver tries to use the simple handler first.

The idea of using specialized/simplified handlers is from the Intel
and Mellanox drivers.

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