net: tc: Limit the max amount of packets in the traffic-class-fifo

To avoid starvation of a traffic class by another, limit the maximum amount
of packets, that can sit in a single traffic-class-fifo to a fraction of
the maximum amount of available packets. In the tx-case also reserve
packets for direct sending, in the case, where
CONFIG_NET_TC_SKIP_FOR_HIGH_PRIO is enabled.

Signed-off-by: Cla Mattia Galliard <cla-mattia.galliard@zuehlke.com>