irq_work, alpha: Fix up arch hooks

Commit e360adbe29 ("irq_work: Add generic hardirq context
callbacks") fouled up the Alpha bit, not properly naming the
arch specific function that raises the 'self-IPI'.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Michael Cree <mcree@orcon.net.nz>
Cc: stable@kernel.org # 37+
Link: http://lkml.kernel.org/n/tip-gukh0txmql2l4thgrekzzbfy@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>