FIX: Prevent attitude stack overflow when GPS connected

I sometimes get an Attitude stack overflow when a GPS is connected that already has acquired satellites while the flight controller is starting. I'm using a Neo-6M, 38400 bps, but I can also reproduce it using the ArduPilot GPS emulator. By increasing the stack by 100 bytes, the minimum remaining stack I observed is 96 bytes. 

I'm using the BrainFPV flight controller, but this issue shouldn't be flight controller specific.