mlxsw: spectrum_router: Fix an IS_ERR() vs NULL check

The mlxsw_sp_crif_alloc() function returns NULL on error.  It doesn't
return error pointers.  Fix the check.

Fixes: 78126cfd ("mlxsw: spectrum_router: Maintain CRIF for fallback loopback RIF")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>