ibmveth: Convert driver specific assert to BUG_ON

We had a driver specific assert function which wasn't enabled most of the
time. Convert them to BUG_ON and enable them all the time.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Santiago Leon <santil@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>