ath6kl: assure headroom of skbuff is writable in .start_xmit()

An issue was found brcmfmac driver in which a skbuff in .start_xmit()
callback was actually cloned. So instead of checking for sufficient
headroom it should also be writable. Hence use skb_cow_head() to
check and expand the headroom appropriately.

Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Tested-by: Steve deRosier <derosier@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>