tg3: Fix loopback tests

The half-duplex bit in the MAC MODE register will be set during the
loopback test if the external link is in half-duplex mode.  This will
cause the loopback test to fail on newer devices.  This patch turns the
half-duplex bit off for the test.

Also, newer devices fail the internal phy loopback test because the phy
link takes a little while to come up.  This patch adds code to wait for
the link before proceeding with the test.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>