CAPWAP : Fix wrong offset for AC Name with Priority

Thanks to Massimo Velluci for sample

Change-Id: Ibab2658c2c11c9940d3fdebb0bdc42752d57e1ae
Reviewed-on: https://code.wireshark.org/review/3871
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>