soundtrigger: fill in default extras from dsp

We seem to get an improper offset when getting the value from the DSP
(offset 100, data size 1), when this happens, return a new phrase event
with valid extras which are initialized to some default values.

Change-Id: Ie041f78fec6bdbcd82286b54a7b7fb0c98c740f5
Signed-off-by: Roman Birg <roman@cyngn.com>