serial: sprd: Fix missing spin_unlock in sprd_handle_irq()

Fix return from sprd_handle_irq() with spin_lock held.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/tty/serial/sprd_serial.c b/drivers/tty/serial/sprd_serial.c
index 594b633..bca975f 100644
--- a/drivers/tty/serial/sprd_serial.c
+++ b/drivers/tty/serial/sprd_serial.c
@@ -293,8 +293,10 @@
 
 	ims = serial_in(port, SPRD_IMSR);
 
-	if (!ims)
+	if (!ims) {
+		spin_unlock(&port->lock);
 		return IRQ_NONE;
+	}
 
 	serial_out(port, SPRD_ICLR, ~0);