tg3: Tune 5785 clock switching

This patch tunes the timeouts the CPMU uses to decide when to switch
from the clocks output by the PHY to internal clock sources.

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>