net: bcmgenet: update bcmgenet_ephy_power_up to clear CK25_DIS bit

The CK25_DIS bit controls whether a 25Mhz clock is fed to the GPHY or
not, in preparation for powering down the integrated GPHY when relevant,
make sure we clear that bit.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>