BugLink: http://bugs.launchpad.net/bugs/1546572

Add a helper to get the max link width of the port.
Similar to the helper to get the max link speed.

v2: fix typo in commit message
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 60d8edd4)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>