floss: Handle mSBC packets without valid header

Blindly dropping all packets without valid header causes the stack to
not send any data as a reply to the controller. This may lead some
controllers to stop sending further traffic.

Bug: 256104713
Tag: #floss
Test: Ran HFP tests and verified HFP WBS works on devices with SCO
packet size 60 and 72

Change-Id: I2d3c4ac7f178185793842e3750f1cba1464d5e49