Bluetooth: btle: Fix reserved field dissection

Also cosmetic changes:
1. Clear Info Column at start (fix garbages from previous protocols like Ubertooth)
2. Refactor a little all expert field definitions

Change-Id: Ie1cba8c1578597f2d896ec75f7d2af4987325bf3
Reviewed-on: https://code.wireshark.org/review/3768
Reviewed-by: Michal Labedzki <michal.labedzki@tieto.com>