mlxsw: Move ethtool_ops to spectrum_ethtool.c

mlxsw: Move ethtool_ops to spectrum_ethtool.c

Add spectrum_ethtool.c file for ethtool code.
Move ethtool_ops and the relevant code from spectrum.c to
spectrum_ethtool.c.

Signed-off-by: Amit Cohen <amitc@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>