net: selftests: Add lanes setting test

Test that setting lanes parameter is working.

Set max speed and max lanes in the list of advertised link modes,
and then try to set max speed with the lanes below max lanes if exists
in the list.

And then, test that setting number of lanes larger than max lanes fails.

Do the above for both autoneg on and off.

$ ./ethtool_lanes.sh

TEST: 4 lanes is autonegotiated [ OK ]
TEST: Lanes number larger than max width is not set [ OK ]
TEST: Autoneg off, 4 lanes detected during force mode [ OK ]
TEST: Lanes number larger than max width is not set [ OK ]

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>