soc: qcom: rpm_stats: Fix snprintf string format

Undefined string lengths could lead to buffer overflows. Format the
string correctly to ensure proper usage of snprintf.

Change-Id: Ia85754327d342aefce2a67eb7f5aeca857d8fbcf
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>