i40e: Make it clear a parameter is never used

Flag the filter_mask parameter as __always_unused in the
ndo_bridge_getlink function.

Change-ID: Ifc1e99c7fb84bcbf81cf7b0ac891ad8ca956ffb2
Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>