[PATCH] ibmveth: Harden driver initilisation

This patch has been floating around for a while now, Santi originally
sent it in March: http://www.spinics.net/lists/netdev/msg00471.html

After a kexec the ibmveth driver will fail when trying to register
with the Hypervisor because the previous kernel has not unregistered.

So if the registration fails, we unregister and then try again.

We don't unconditionally unregister, because we don't want to disturb
the regular code path for 99% of users.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Santiago Leon <santil@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>