ixgbe: fix multi-process support

ixgbe: fix multi-process support

When using multiple processes, the TX function used in all processes
should be the same, otherwise the secondary processes cannot transmit
more than tx-ring-size - 1 packets.
To achieve this, we extract out the code to select the ixgbe TX function
to be used into a separate function inside the ixgbe driver, and call
that from a secondary process when it is attaching to an
already-configured NIC.

Testing with symmetric MP app shows that we are able to RX and TX from
both primary and secondary processes once this patch is applied.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Reshma Pattan <reshma.pattan@intel.com>