selftests: mlxsw: ethtool_lanes: Source ethtool lib from correct path

selftests: mlxsw: ethtool_lanes: Source ethtool lib from correct path

Source the ethtool library from the correct path and avoid the following
error:

./ethtool_lanes.sh: line 14: ./../../../net/forwarding/ethtool_lib.sh: No such file or directory

Fixes: 40d269c000bd ("selftests: forwarding: Move several selftests")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/2112faff02e536e1ac14beb4c2be09c9574b90ae.1724150067.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>