wil6210: prefetch head of packet

As soon as skb is ready to be reaped, prefetch 1-st cache line.
This accelerates data access that is performed later, during the
packet classification by the driver and IP stack.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>