libeffects: Fix processed samples for downmix effect

In downmix effect, input and output sizes can be different,
where the output produced follows:
outputSize = (inputSize * outFrameSize/inFrameSize)

Writing a size beyond the capacity of FMQ fails the write
operation leading to no sound.

Fix the generated output sample in above mentioned ratio.

Bug: 318926783
Test: play a 8 channel file and check sound

Change-Id: Ifbe6fba90d08a2e30339117f90b994cfdb668bb4