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>