openrisc: really pass correct arg to schedule_tail

Commit 287ad220 tried to set up the argument
to schedule_tail, but ended up using TI_STACK which isn't a defined symbol.
Sadly, the old openrisc compiler silently ignores this fact and it was first
discovered now when building with an updated toolchain.
Reported-by: Christian Svensson <blue@cmd.nu>
Signed-off-by: Jonas Bonn <jonas@southpole.se>

arch/openrisc/kernel/entry.S

@@ -1050,7 +1050,7 @@ ENTRY(_switch)
 	 * we are expected to have set up the arg to schedule_tail already,
 	 * hence we do so here unconditionally:
 	 */
-	l.lwz   r3,TI_STACK(r3)		/* Load 'prev' as schedule_tail arg */
+	l.lwz   r3,TI_TASK(r3)		/* Load 'prev' as schedule_tail arg */
 	l.jr	r9
 	 l.nop