carl9170: fix tx_ampdu_upload counter

tx_ampdu_upload was not decreased when an a-MPDU
frame had to be kicked out from the tx_pending
queues.

This broke ampdu aggregation, because the scheduler
waits until tx_ampdu_upload drops to zero, before
making the next aggregate.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>