ibmveth: Fix max MTU limit

The max MTU limit defined for ibmveth is not accounting for
virtual ethernet buffer overhead, which is twenty-two additional
bytes set aside for the ethernet header and eight additional bytes
of an opaque handle reserved for use by the hypervisor. Update the
max MTU to reflect this overhead.

Fixes: d894be57 ("ethernet: use net core MTU range checking in more drivers")
Fixes: 110447f8 ("ethernet: fix min/max MTU typos")
Signed-off-by: Thomas Falcon <tlfalcon@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>