net/ixgbe: do not start on unsupported loopback mode

net/ixgbe: do not start on unsupported loopback mode

Only TX->RX loopback is supported currently on 82599EB. If a user wants
to apply an another loopback configuration (!= IXGBE_LPBK_82599_TX_RX),
ixgbe PMD ignores it and continues the configuration without raising
any error.

Let's increase robustness of this part by checking if the requested
loopback mode is correct for the current device, before starting it.
If it is not valid, PMD will refuse to start.

Signed-off-by: Julien Meunier <julien.meunier@nokia.com>
Acked-by: Wei Zhao <wei.zhao1@intel.com>