bnxt_en: Fix occasional ethtool -t loopback test failures

In the current code, we setup the port to PHY or MAC loopback mode
and then transmit a test broadcast packet for the loopback test.  This
scheme fails sometime if the port is shared with management firmware
that can also send packets.  The driver may receive the management
firmware's packet and the test will fail when the contents don't
match the test packet.

Change the test packet to use it's own MAC address as the destination
and setup the port to only receive it's own MAC address.  This should
filter out other packets sent by management firmware.

Fixes: 91725d89

 ("bnxt_en: Add PHY loopback to ethtool self-test.")
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>