mlxsw: spectrum: Change mlxsw_sp_upper to LAG structure

The structure mlxsw_sp_upper is used only as LAG. Rename it to
mlxsw_sp_lag and move it to spectrum.c file, as it is used only there.
Move the function mlxsw_sp_lag_get() with the structure.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>