msm: qdsp5: Initialize event length during adsp fake event generation

During adsp fake event generation, event length is not getting
initialized. This sometimes results in incorrect event data being read
during fake event processing by adsp driver clients.

Fix this by initializing event length properly before generating the
fake event.

Change-Id: Ifcab5adf542b04eef3dc03913b8e10c68f686e8c
Signed-off-by: Manish Dewangan <manish@codeaurora.org>