MIP6: Access network identifier option is not full decoded

* Add subtype Geo Location (2) and Operator Identifier (3)
* Use also offset to calculate the length
* Remove proto_tree_add_text

Issue reported by Ravindranath K M

Bug:10712
Change-Id: I9ad81ece856df134f3a706ad774d34e638cb5c94
Reviewed-on: https://code.wireshark.org/review/5397
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>