msm:ipa: Prevent NAT table deletion only if public ip is not assigned

Currnetly NAT table is not deleted even if public ip is assigned to
NAT table. Add check to prevent deletion only if public ip is not assigned.

Change-Id: I4855b21472d3f6bf541d07733b18592e9e677ce6
Acked-by: Pooja Kumari <kumarip@qti.qualcomm.com>
Signed-off-by: Mohammed Javid <mjavid@codeaurora.org>