iavf: Add change MTU message

Add a netdev_dbg log entry in case of a change of MTU so that user is
notified about this change in the same manner as in case of pf driver.

Signed-off-by: Patryk Małek <patryk.malek@intel.com>
Tested-by: George Kuruvinakunnel <george.kuruvinakunnel@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>