cbus: Fix nested IRQ acking

chip->irq_ack is never called for nested threaded IRQs, because they
don't have a primary IRQ handler.
So we must ACK the IRQs in the toplevel retu and tahvo IRQ handler threads.

Signed-off-by: Michael Buesch <m@bues.ch>
Signed-off-by: Tony Lindgren <tony@atomide.com>