mwifiex: bridged packets cause wmm_tx_pending counter to go negative

When a packet is queued from the bridge, wmm_tx_pending is not
incremented, but when the packet is dequeued the counter is decremented.

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