ath6kl: Fix memory leak of rx packets in endpoint 0

htc_packet and htc_packet->buf_start are separately allocated
for endpoint 0. This is different for other endpoints where
packets are allocated as skb where htc_packet is skb->head
and they are freed properly. Free htc_packet and htc_packet->buf_start
separatly for endpoint 0.

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