sch_cake: add RFC 8622 LE PHB support to CAKE diffserv handling

Change tin mapping on diffserv3, 4 & 8 for LE PHB support, in essence
making LE a member of the Bulk tin.

Bulk has the least priority and minimum of 1/16th total bandwidth in the
face of higher priority traffic.

NB: Diffserv 3 & 4 swap tin 0 & 1 priorities from the default order as
found in diffserv8, in case anyone is wondering why it looks a bit odd.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
[ reword commit message slightly ]
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>