tg3: Fix AC131 loopback test errors for 5785

The AC131 does not enable the forced transmit clock settings
immediately.  The workaround is to read the register again to get the
setting to take effect.

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