wl12xx: fix tx_queue_count spurious increment

Only increment the queue count after actually queuing the skb. This
avoids a spurious increment is case of dropped packets.

Also move the Tx-watermark checking code after the packet is enqueued.
This makes the count more accurate - it includes the just-queued
packet.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>