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

[ Upstream commit f8669d7b5f5d2d88959456ae9123d8bb6fdc1ebe ]

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: 40d269c0

 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>