msm: ipa: fix debugfs for hw stats

Fix debugfs behaviour for IPA HW stats to handle
the case where stats are not configured.

Change-Id: I4ef446f1dff969c4f7013beef28dc042016bb876
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>