diag: Replace improper checks with proper ones to clear stats

Invalid checks to reset the debugfs stats causes invalid behaviour
in using debug structures. Use proper check to effect the proper
clearing of the debug stats.

Change-Id: I29e6f3d8909914a506337f4e788b521439e9d69b
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>