mlxsw: spectrum_ethtool: Extract a helper to get Ethernet attributes

In order to allow reusing the logic, extract from
mlxsw_sp_port_get_link_ksettings() the code to obtain Ethernet protocol
attributes, mlxsw_sp_port_ptys_query().

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>