[capwap] fix wtp_stats_event
[capwap] fix wtp_stats_event

Summary:
fixes wtp interrim accounting.
solves T553

Test Plan: read, install in int or tua

Reviewers: aschultz, druss, hsychla

Reviewed By: aschultz

CC: kreer, thintze, mkraemer, druss, aschultz

Maniphest Tasks: T553

Differential Revision: https://review.tpip.net/D1510

travelping/capwap,travelping/capwap