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: 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.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/drivers/net/mlxsw/ethtool_lanes.sh

@@ -2,6 +2,7 @@
 # SPDX-License-Identifier: GPL-2.0
 
 lib_dir=$(dirname $0)/../../../net/forwarding
+ethtool_lib_dir=$(dirname $0)/../hw
 
 ALL_TESTS="
 	autoneg
@@ -11,7 +12,7 @@ ALL_TESTS="
 NUM_NETIFS=2
 : ${TIMEOUT:=30000} # ms
 source $lib_dir/lib.sh
-source $lib_dir/ethtool_lib.sh
+source $ethtool_lib_dir/ethtool_lib.sh
 
 setup_prepare()
 {