In ifconfig athX stats, tx packets and bytes were getting double
incremented in NSS offload disabled case. It was due to tx.comp_pkt
stats getting updated both in per packet Tx completion path and in
Tx per ppdu indication from HTT path.
Fix by updating stats only in the Tx. completion path.
Change-Id: Id75d914874480a4e24ccb13223818b67869eb378