msm: npu: Only initialize completion variable if necessary

It is possible that the completion variable will be reinitialized
by mistake in race condition. This change is to only reinitialize
it before sending down to firmware via IPC.

Change-Id: I6e437fd431889a6dce3eac838ee044af9a06348c
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>