net: phy: broadcom: Only advertise EEE for supported modes

We should not be advertising EEE for modes that we do not support,
correct that oversight by looking at the PHY device supported linkmodes.

Fixes: 99cec8a4 ("net: phy: broadcom: Allow enabling or disabling of EEE")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>