serial/nwpserial: Fix wrong register read address and add interrupt acknowledge.

The receive interrupt routine checks the wrong register if the
receive fifo is empty. Further an explicit interrupt acknowledge
write is introduced. In some circumstances another interrupt was
issued.
Signed-off-by: NBenjamin Krill <ben@codiert.org>
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

drivers/serial/nwpserial.c

@@ -145,11 +145,13 @@ static irqreturn_t nwpserial_interrupt(int irq, void *dev_id)
 		ch = dcr_read(up->dcr_host, UART_RX);
 		if (up->port.ignore_status_mask != NWPSERIAL_STATUS_RXVALID)
 			tty_insert_flip_char(tty, ch, TTY_NORMAL);
-	} while (dcr_read(up->dcr_host, UART_RX) & UART_LSR_DR);
+	} while (dcr_read(up->dcr_host, UART_LSR) & UART_LSR_DR);
 
 	tty_flip_buffer_push(tty);
 	ret = IRQ_HANDLED;
 
+	/* clear interrupt */
+	dcr_write(up->dcr_host, UART_IIR, 1);
 out:
 	spin_unlock(&up->port.lock);
 	return ret;