can: pcan_usb: fix wrong memcpy() bytes length

Fix possibly wrong memcpy() bytes length since some CAN records received from
PCAN-USB could define a DLC field in range [9..15].
In that case, the real DLC value MUST be used to move forward the record pointer
but, only 8 bytes max. MUST be copied into the data field of the struct
can_frame object of the skb given to the network core.

Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>