powerpc/vio: Add missing unlock in error path

Add an unlock before exiting the function.

A simplified version of the semantic patch that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@r exists@
expression E1;
identifier f;
@@

f (...) { <+...
* spin_lock_irq (E1,...);
... when != E1
* return ...;
...+> }
// </smpl>
Signed-off-by: Julia Lawall <julia@diku.dk>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/vio.c

@@ -645,8 +645,10 @@ void vio_cmo_set_dev_desired(struct vio_dev *viodev, size_t desired)
 			found = 1;
 			break;
 		}
-	if (!found)
+	if (!found) {
+		spin_unlock_irqrestore(&vio_cmo.lock, flags);
 		return;
+	}
 
 	/* Increase/decrease in desired device entitlement */
 	if (desired >= viodev->cmo.desired) {