sfc: Encapsulate access to efx_{channel,nic}::last_irq_cpu in self-test

Cleanup in preparation for doing an event test on ifup.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/ethernet/sfc/nic.c b/drivers/net/ethernet/sfc/nic.c
index 5da8af5..4c47b75 100644
--- a/drivers/net/ethernet/sfc/nic.c
+++ b/drivers/net/ethernet/sfc/nic.c
@@ -1332,8 +1332,10 @@
 }
 
 
-void efx_nic_generate_test_event(struct efx_channel *channel)
+void efx_nic_event_test_start(struct efx_channel *channel)
 {
+	channel->last_irq_cpu = -1;
+	smp_wmb();
 	efx_magic_event(channel, EFX_CHANNEL_MAGIC_TEST(channel));
 }
 
@@ -1382,8 +1384,10 @@
  * Interrupt must already have been enabled, otherwise nasty things
  * may happen.
  */
-void efx_nic_generate_interrupt(struct efx_nic *efx)
+void efx_nic_irq_test_start(struct efx_nic *efx)
 {
+	efx->last_irq_cpu = -1;
+	smp_wmb();
 	efx_nic_interrupts(efx, true, true);
 }