This might be fixed in the Ubuntu 15.04 "Vivid Vervet" kernel 3.19.0-17.17 by this commit:
iwlwifi: fix max_ht_ampdu_exponent for older devices
The diff file mentions your Centrino Advanced-N 6235.
This might be fixed in the Ubuntu 15.04 "Vivid Vervet" kernel 3.19.0-17.17 by this commit:
iwlwifi: fix max_ht_ ampdu_exponent for older devices
The diff file mentions your Centrino Advanced-N 6235.