mlxsw: spectrum_router: Do not query MAX_VRS on each iteration

MLXSW_CORE_RES_GET involves a call to spectrum_core, a separate module.
Instead of making the call on every iteration, cache it up front, and use
the value.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>