[TG3]: Prescaler fix

Internal hardware timers become inaccurate after link events.  Clock
frequency switches performed by the CPMU fail to adjust timer
prescalers.  The fix is to detect core clock frequency changes during
link events and adjust the timer prescalers accordingly.

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