bnxt_en: Refactor tx completion path.

XDP_TX requires a different function to handle completion.  Add a
function pointer to handle tx completion logic.  Regular TX rings
will be assigned the current bnxt_tx_int() for the ->tx_int()
function pointer.

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