mlxsw: spectrum: Query max_lag once

The maximum number of LAGs is queried from core several times. It is
used to allocate LAG array, and then to iterate over it. In addition, it
is used for PGT initialization. To simplify the code, instead of
querying it several times, store the value as part of 'mlxsw_sp' and use
it.

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>