net: socionext: clear rx irq correctly

commit 63ae7949e94a ("net: socionext: Use descriptor info instead of MMIO reads on Rx")
removed constant mmio reads from the driver and started using a descriptor
field to check if packet should be processed.
This lead the napi rx handler being constantly called while no packets
needed processing and ksoftirq getting 100% cpu usage. Issue one mmio read
to clear the irq correcty after processing packets

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/socionext/netsec.c

@@ -735,8 +735,11 @@ static int netsec_process_rx(struct netsec_priv *priv, int budget)
 		u16 idx = dring->tail;
 		struct netsec_de *de = dring->vaddr + (DESC_SZ * idx);
 
-		if (de->attr & (1U << NETSEC_RX_PKT_OWN_FIELD))
+		if (de->attr & (1U << NETSEC_RX_PKT_OWN_FIELD)) {
+			/* reading the register clears the irq */
+			netsec_read(priv, NETSEC_REG_NRM_RX_PKTCNT);
 			break;
+		}
 
 		/* This  barrier is needed to keep us from reading
 		 * any other fields out of the netsec_de until we have