bnxt_en: Add ethtool mac loopback self test.

bnxt_en: Add ethtool mac loopback self test.

Orabug: 26402533, 26325599, 26366387

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>
(cherry picked from commit f7dc1ea6c4c1f31371b7098d6fae0d49dc6cdff1)
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c
	drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.h
Signed-off-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>