bnx2x: loopback test failure

A link change interrupt might be queued and activated after the loopback was set
and it will cause the loopback to fail. The PHY lock should be kept until the
loopback test is over.

That implies that the bnx2x_test_link should used within the loopback function
and not bnx2x_wait_for_link since that function also takes the PHY link

Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>