ixgbe: add Tx->Rx loopback mode for 82599

ixgbe: add Tx->Rx loopback mode for 82599

82599 has two loopback operation modes, Tx->Rx and Rx->Tx.
For the time being only Tx->Rx is supported.

The new field lpbk_mode added in struct rte_eth_conf defines loopback
operation mode for certain ethernet controller. By default the value
of lpbk_mode is 0, meaning loopback mode disabled.

Since each ethernet controller has its own definition of loopback modes,
API user has to check both datasheet and implementation of certain driver
so as to understand what are valid values to be set, and what are the
expected behaviors.

Check IXGBE_LPBK_82599_XXX which are defined in ixgbe_ethdev.h
for valid values of 82599 loopback mode.

Signed-off-by: Qinglai Xiao <jigsaw@gmail.com>
Acked-by: Ivan Boule <ivan.boule@6wind.com>
Acked-by: Venky Venkatesan <venky.venkatesan@intel.com>