ath6kl: Remove unnecessary retrieval of first list entry in ath6kl_htc_tx_setup_scat_list()

It is unnecessary to take the first list entry from queue again for
transmission. Sometimes it may look racy when the head of the list
changes between subsequent retrival, but should not happen in
practical.

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