cpuidle: lpm-levels: Fix snprintf string format

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

Change-Id: I996f41197606f4b9877cf0fbc6c57145480b5c1c
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>