STPK-1429 Fix erroneous buffer size

STPK-1429 Fix erroneous buffer size

Not sure what I had in mind with that multiplication. Anyway, the
buffer is zero-initialized, so keep it at a reasonable size, which
is to say... 32. That's plenty enough for now, as the maximum we
encountered is 20 (accel x,y,z 16 bits + gyro x,y,z + 64 bits ts).

We don't need the expected data size calculation, because the
values are set to zero by default, and because that sum would not
be correct anyway in the case where fields such as the timestamp
are shared between sensors. Plus, we were not even verifying that
the computed sum did really fit our buffer.

Issue: STPK-1429

Change-Id: Ie001ceeb634866a1460444bb839918045b72de31
Signed-off-by: Patrick Porlan <patrick.porlan@intel.com>