Merge pull request #135 from andrew-elder/staging

lib: controller: add mutex around endpoint vector lookup
small change, fixed bug that was not reading the highest descriptor index information for multiple index descriptor types
AVB_interface descriptor was not being read
Merge pull request #136 from vronchetti/staging

Bug Fixes
lib: controller: function end_station_imp *at(size_t i) now uses lock_guard

Previous implementation used a plain mutex, which could be left in a locked state
if the .at(i) operation caused an exception.