mlxsw: spectrum_qdisc: Track tclass_num as int, not u8

tclass_num is just a number, a value that would be ordinarily passed around
as an int. (Which is unlike a u8 prio_bitmap.) In several places,
tclass_num already is an int. Convert the remaining instances.

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