serial: drivers/serial/pmac_zilog.c: add missing unlock

In an error handling case the lock is not unlocked.

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_irqsave (E1,...);
... when != E1
* return ...;
...+> }
// </smpl>

Signed-off-by: Julia Lawall <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/serial/pmac_zilog.c

···
 		uap->curregs[R5] = new_reg;
 
 		/* NOTE: Not subject to 'transmitter active' rule. */
-		if (ZS_IS_ASLEEP(uap))
+		if (ZS_IS_ASLEEP(uap)) {
+			spin_unlock_irqrestore(&port->lock, flags);
 			return;
+		}
 		write_zsreg(uap, R5, uap->curregs[R5]);
 	}