iavf: sprinkle netdev_assert_locked() annotations

Lockdep annotations help in general, but here it is extra good, as next
commit will remove crit lock.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>