msm:ipa4: Fix to return error if nat table memory already allocated

Because of returning the success memory already allocated
case causing the pdn memory allocated multiple times in the
calling function and never get freed. Added to changes to
return error if nat table memory already allocated.

Change-Id: Ie64f1e4ec8862dc9c6f3c2a0a42d9eb9f8c21a26
Acked-by: Ashok Vuyyuru <avuyyuru@qti.qualcomm.com>
Signed-off-by: Mohammed Javid <mjavid@codeaurora.org>