mlxsw: spectrum_trap: Use 'size_t' for array sizes

Use 'size_t' instead of 'u64' for array sizes, as this this is correct
type to use for expressions involving sizeof().

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