net: dsa: bcm_sf2: remove unneeded supported flags

The SF2 driver is masking the supported bitfield of its private copy of
the ports' ethtool_eee structures. It is used nowhere, thus remove it.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>