ibmvnic: Update MTU after device initialization

It is possible for the MTU to be changed during the initialization
process with the VNIC Server.  Ensure that the net device is updated
to reflect the new MTU.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>