mlxsw: spectrum: Remove mlxsw_sp_lag_get()

A next patch will add mlxsw_sp_lag_{get,put}() functions to handle LAG
reference counting and create/destroy it only for first user/last user.
Remove mlxsw_sp_lag_get() function and access LAG array directly.

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>