dsp: Address information leak in dsp drivers

Due to uninitialised variables used in q6voice
and q6usm drivers, there is a possibility of stack
information leak whenever lesser size data is copied.
Fix is to address this.

Change-Id: I6888b468ce50114be3139111ccf4696827540cfd
Signed-off-by: Soumya Managoli <quic_c_smanag@quicinc.com>