mlxsw: spectrum_qdisc: Don't use variable array in mlxsw_sp_tclass_congestion_enable

Resolve the sparse warning:
"sparse: Variable length array is used."
Use 2 arrays for 2 PRM register accesses.

Fixes: 96f17e07

 ("mlxsw: spectrum: Support RED qdisc offload")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>