can: peak_usb: fix hwtstamp assignment

The skb->tstamp is set to the hardware timestamp when available in the USB
urb message. This leads to user visible timestamps which contain the 'uptime'
of the USB adapter - and not the usual system generated timestamp.

Fix this wrong assignment by applying the available hardware timestamp to the
skb_shared_hwtstamps data structure - which is intended for this purpose.

Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>