bnxt_en: Refactor __bnxt_xmit_xdp().

__bnxt_xmit_xdp() is used by XDP_TX and ethtool loopback packet transmit.
Refactor it so that it can be re-used by the XDP_REDIRECT logic.
Restructure the TX interrupt handler logic to cleanly separate XDP_TX
logic in preparation for XDP_REDIRECT.

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