[TG3]: 5784 / 5764 GPHY power down fix

5784 and 5764 devices fail to link / pass traffic after one load /
unload cycle.  This happens because of a hardware bug in the new CPMU.
During normal operation, the MAC depends on the PHY clock being
available.  When the PHY is powered down, the clock the MAC depends on
is disabled.  The fix is to switch the MAC clock to an alternate source
before powering down the PHY, and to restore the MAC clock to the PHY
source upon device resume.

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>