net: bcmgenet: ensure EXT_ENERGY_DET_MASK is clear

Setting the EXT_ENERGY_DET_MASK bit allows the port energy detection
logic of the internal PHY to prevent the system from sleeping. Some
internal PHYs will report that energy is detected when the network
interface is closed which can prevent the system from going to sleep
if WoL is enabled when the interface is brought down.

Since the driver does not support waking the system on this logic,
this commit clears the bit whenever the internal PHY is powered up
and the other logic for manipulating the bit is removed since it
serves no useful function.

Fixes: 1c1008c7 ("net: bcmgenet: add main driver file")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>