ionic: get MTU from lif identity

Change from using hardcoded MTU limits and instead use the
firmware defined limits. The value from the LIF attributes is
the frame size, so we take off the header size to convert to
MTU size.

Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>