can: kvaser_usb: Fix potential uninitialized variable use

If alloc_can_err_skb() fails, cf is never initialized.
Move assignment of cf inside check.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jimmy Assarsson <jimmyassarsson@gmail.com>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>