CC3D: Bump manual control stack size 50

Reports of stack overflow when using rate mode.  Free stack when
not being used before this was 100 but it could go lower when
various modes are engaged.

Heap remaining for a standard configuration after this patch is 2880 so it should
work fine.

For testing purposes here is a diagnostics enabled version of CC3D without this patch:
https://dl.dropboxusercontent.com/u/6645063/fw_coptercontrol_diagnostics.tlfw