i40e: Relax i40e_xsk_wakeup's return value when PF is busy

Return -EAGAIN instead of -ENETDOWN to provide a slightly milder
information to user space so that an application will know to retry the
syscall when __I40E_CONFIG_BUSY bit is set on pf->state.

Fixes: b3873a5b ("net/i40e: Fix concurrency issues between config flow and XSK")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/20200205045834.56795-2-maciej.fijalkowski@intel.com

drivers/net/ethernet/intel/i40e/i40e_xsk.c

@@ -791,7 +791,7 @@ int i40e_xsk_wakeup(struct net_device *dev, u32 queue_id, u32 flags)
 	struct i40e_ring *ring;
 
 	if (test_bit(__I40E_CONFIG_BUSY, pf->state))
-		return -ENETDOWN;
+		return -EAGAIN;
 
 	if (test_bit(__I40E_VSI_DOWN, vsi->state))
 		return -ENETDOWN;