[IA64] Stop using the deprecated __do_IRQ() code path

Thomas Gleixner <tglx@linutronix.de> wrote:
>__do_IRQ() has been deprecated after a two years migration phase in
>commit 0e57aa11. Since then another 18 months have gone by ...

Mostly trivial stuff for this. The only tricky part was realizing
that the new handler_*_irq() paths do not use desc->chip->end(irq).
Not a problem for the edge case as the ia64 iosapic routine for
that was nop(). But the "level" case handled interrupt migration
there.  Just use a slightly modified version of the "end" routine
as "unmask" for the level triggered case.

Signed-off-by: Tony Luck <tony.luck@intel.com>