sgi-xp: separate chctl_flags from XPC's notify IRQ

Tie current IPI references to either XPC's notify IRQ or channel control
flags.

Signed-off-by: Dean Nelson <dcn@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>