ath10k: fix return value check in wake_tx_q op

ath10k_mac_tx_push_txq returns either a postive integer (length) on
success or a negative error code on error.

The "if (ret) break;" statement will thus always break out of the loop
immediately after ath10k_mac_tx_push_txq has returned (making the loop
pointless).

A side effect of this fix is that we will iterate the queue until
ath10k_mac_tx_push_txq returns -ENOENT. This will make sure the queue is
not added back to ar->txqs when it is empty. This could potentially
improve performance somewhat (I have seen a small improvement with SDIO
devices).

Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>