mlxsw: spectrum_qdisc: Extract two helpers for handling future FIFOs

Extract from __mlxsw_sp_qdisc_ets_replace() two helpers for handling of one
future FIFO resp. reinitializing the array of future FIFOs.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>