CAPWAP: Add new ett variable

Avoid to have all subtree has expended

Only 2 ett actually and there is > 20 subtree...

Change-Id: Ia4af85345a6f1963d24d1fd589ff8ee09a8ef412
Reviewed-on: https://code.wireshark.org/review/3955
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>