floss: Fix mSBC encoder

The two bytes after sync word should be reserved as zero based on HFP
specification. Setting non-zero value to them may cause HF side not able
to decode the encoded mSBC packet correctly.

Bug: 236413634
Tag: #floss
Test: Build and verify with devices supporting WBS

Change-Id: Ia9a71eb5243ed4b9b335a3b9006d650dc00bf2ca