msm: ipa: Fix to check only reset IPA stats can have data as NULL

Only reset IPA stats can have tether_stat
data as NULL. Query of of tether stat without
allocating memory for structure
wan_ioctl_query_tether_stats should fail.

Change-Id: Iab0fa98ffdcb2b32d75f4e8cb88f250002d2a787
Acked-by: Pooja Kumari <kumarip@qti.qualcomm.com>
Signed-off-by: Mohammed Javid <mjavid@codeaurora.org>