ASoC: qdsp6: Fix device crash issue in LPA seek.

Accessing buffer pointer before initialization results in device crash.
To memset the buffer using physical address also results in a device crash.
Fixed this by initializing the buffer pointer before it is accessed and
memset the buffer using the virtual address.

Change-Id: I3b03f56cf988c9471c7988665bcec3c467e60bfc
Signed-off-by: Amal Paul <amal@codeaurora.org>