net: dsa: mv88e6xxx: add eeprom ops

Remove EEPROM flags in favor of new {get,set}_eeprom chip-wide
functions in the mv88e6xxx_ops structure.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>