ath6kl: cleanup callbacks for different scatter gather method

Define a hook in ath6kl_hif_ops for hif scatter gather mechanism.
When virtual scatter gather is used, call the respective function
directly.

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