mlxsw: core: Move ethtool module callbacks to a common location

Move the implementation of ethtool module callbacks - .get_module_info()
and .get_module_eeprom() - to a common location to allow reuse by the
different mlxsw drivers.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>