i40e: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

This patch fixes the following warning:

drivers/net/ethernet/intel/i40e/i40e_xsk.c: In function ‘i40e_run_xdp_zc’:
drivers/net/ethernet/intel/i40e/i40e_xsk.c:217:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   bpf_warn_invalid_xdp_action(act);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/intel/i40e/i40e_xsk.c:218:2: note: here
  case XDP_ABORTED:
  ^~~~

Signed-off-by: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>