nfp: process MTU updates from firmware flower app

Now that control message processing occurs in a workqueue rather than a BH
handler MTU updates received from the firmware may be safely processed.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>