sfc: fix kernel-doc on struct efx_loopback_state

Missing 'struct' keyword caused "cannot understand function prototype"
 warnings.
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/selftest.c

@@ -67,7 +67,7 @@ static const char *const efx_interrupt_mode_names[] = {
 	STRING_TABLE_LOOKUP(efx->interrupt_mode, efx_interrupt_mode)
 
 /**
- * efx_loopback_state - persistent state during a loopback selftest
+ * struct efx_loopback_state - persistent state during a loopback selftest
  * @flush:		Drop all packets in efx_loopback_rx_packet
  * @packet_count:	Number of packets being used in this test
  * @skbs:		An array of skbs transmitted