sc16is7xx: Don't spin if no data received

See: #2676

Signed-off-by: Phil Elwell <[email protected]>

Author: Phil Elwell

drivers/tty/serial/sc16is7xx.c

@@ -696,6 +696,8 @@ static bool sc16is7xx_port_irq(struct sc16is7xx_port *s, int portno)
 			rxlen = sc16is7xx_port_read(port, SC16IS7XX_RXLVL_REG);
 			if (rxlen)
 				sc16is7xx_handle_rx(port, rxlen, iir);
+			else
+				return false;
 			break;
 		case SC16IS7XX_IIR_THRI_SRC:
 			sc16is7xx_handle_tx(port);