mlxsw: spectrum_fid: Make flood index calculation more robust

802.1D FIDs use a per-FID flood table, where the flood index into the
table is calculated by subtracting 4K from the FID's index.

Currently, 802.1D FIDs start at 4K, so the calculation is correct, but
if it was ever to change, the calculation will no longer be correct.

In addition, this change will allow us to reuse the flood index
calculation function in the next patch, where we are going to emulate
802.1Q FIDs using 802.1D FIDs.

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