bnxt_en: Use short TX BDs for the XDP TX ring.

No offload is performed on the XDP_TX ring so we can use the short TX
BDs.  This has the effect of doubling the size of the XDP TX ring so
that it now matches the size of the rx ring by default.

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