mwifiex: transmit packet stats incorrect.

tx_packets counter is incremented for aggregated packets, when it had
already been incremented for the aggregated packet's constituent
parts. Removing the extra count.

Signed-off-by: Marty Faltesek <mfaltesek@google.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>