net: stats: Add traffic class statistics

Add statistics for number of packets and bytes to each traffic
class. Print this information in net-shell.

Also make sure that we do not calculate total packet length many
times. So calculate network packet total length once and then use
that value instead of calculating it many times in a row.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>