Impossible to increase Roll/Pitch P value

**Impossible to increase Stab Roll/Pitch P value**
Steps to reproduce:
1. Connect APM, go to Extended tuning.
2. Lock Pitch and Roll values
3. Decrease Stabilize Roll P (in my case to 2.5)
4. press Write params
5. press Refresh params
Result : now it's impossible to increase it back neither with spinner buttons, neither with entering from keyboard. Only decrease allowed.
Expected: increase is allowed back to max value limit

**Same behaviour with Rate Roll P and Rate Pitch P**
Once decreased and write params it can not be increased back.

Mission planner version (today updated BETA):
MP v1.3.37 build 1.1.5968.13772
Firmware:
Frame: QUAD
ArduCopter V3.2 (c8e0f3e1)
Hardware:
miniApm 3.1

Start by reproducing the issue in Extended tuning with the listed Mission Planner and ArduCopter versions, focusing on parameter editing after Write params and Refresh params. The fix is complete when Stabilize Roll/Pitch P and Rate Roll/Pitch P can be increased again up to their maximum after being decreased and saved.

Written by the indexing model from the issue text.