ath6kl: Claim sdio function only at appropriate places

There are places where tx_complete callbacks are called with
claiming the sdio function. It is not necessary to hold the
sdio func for longer. This may even affect the host side power
save, if it is supported by the controller.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>