msm: camera: isp: Initialize used_bytes to avoid corruption

UBWC update does not update KMD cmd buffer, hence used_bytes
should be 0. But, due to uninitialized garbage value the
hw_update_entries is corrupted, causing CDM Invalid command error.
Initialize used_bytes to 0, to fix this issue.

Change-Id: I0f8a77f782c833623af5773d15a5c4ce84492125
Signed-off-by: Harsh Shah <harshs@codeaurora.org>