crypto: msm: check length before copying to buf in _debug_stats_read

Make sure that `len` is not larger than `count` before copying data
to userspace `buf` in _debug_stats_read().

Change-Id: Iafb7cfa3828653f8c28183c812797c3d9a183da1
Signed-off-by: Zhen Kong <zkong@codeaurora.org>