soc: qcom: rq_stats: use uint64_t instead of int64_t for timer diff

Assume time will not fall back and then there is no
need for checking if timer diff is < 0. So just
use uint64_t instead of int64_t for timer diff
to avoid the compiler warning of comparison of
distinct pointer types lacks a cast error.

Change-Id: I272de6d889cd5d9b6205c7dfb783883dd7060caf
Signed-off-by: Maria Yu <aiquny@codeaurora.org>