i40e: fix a bug in debugfs with add/del macaddr

The new code flow requires us to grab the filter list lock before
adding/deleting the filter.

Change-ID: I4eaef508ab4da2d1b2e23f20f2a78d931d5b6aeb
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>