bnxt_en: Add ethtool mac loopback self test.

The mac loopback self test operates in polling mode.  To support that,
we need to add functions to open and close the NIC half way.  The half
open mode allows the rings to operate without IRQ and NAPI.  We
use the XDP transmit function to send the loopback packet.

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