drivers: qcom: lpm-stats: Fix undefined access error

In cleanup_stats(), a freed memory pointer pos might be accessed for
list traversal. Switch to using _safe() variant of the list API to
prevent undefined accesses.

Change-Id: I7d068cb7813ccb9bfdbcab4646b4ec890145828a
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>