nfp: modify app MTU setting callbacks

Rename the 'change_mtu' app callback to 'check_mtu'. This is called
whenever an MTU change is requested on a netdev. It can reject the
change but is not responsible for implementing it.

Introduce a new 'repr_change_mtu' app callback that is hit when the MTU
of a repr is to be changed. This is responsible for performing the MTU
change and verifying it.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>