can: vxcan: improve handling of missing peer name attribute

Picking up the patch from Serhey Popovych (commit 191cdb38,
"veth: Be more robust on network device creation when no attributes").

When the peer name attribute is not provided the former implementation tries
to register the given device name twice ... which leads to -EEXIST.
If only one device name is given apply an automatic generated and valid name
for the peer.

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