event/sw: fix credit tracking in port dequeue

event/sw: fix credit tracking in port dequeue

Single-link optimized ports previously did not correctly track
credits when dequeued, and re-enqueued as a FORWARD type. This
could "inflate" the number of credits in the system.

A unit test is added to reproduce and verify the issue, and the
fixed implementation counts FORWARD packets, and reduces the
number of credits the port has if it is of single-link type.

Fixes: 656af9180014 ("event/sw: add worker core functions")

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Gage Eads <gage.eads@intel.com>