bnxt_en: Add a set of TX rings to support XDP.

Add logic for an extra set of TX rings for XDP.  If enabled, this
set of TX rings equals the number of RX rings and shares the same
IRQ as the RX ring set.  A new field bp->tx_nr_rings_xdp is added
to keep track of these TX XDP rings.  Adjust all other relevant functions
to handle bp->tx_nr_rings_xdp.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>