pass struct irq_desc * to all other IRQ accessors

pass struct irq_desc * to all other IRQ accessors

This is again because the descriptor is generally more useful (with
the IRQ number being accessible in it if necessary) and going forward
will hopefully allow to remove all direct accesses to the IRQ
descriptor array, in turn making it possible to make this some other,
more efficient data structure.

This additionally makes the .end() accessor optional, noting that in a
number of cases the functions were empty.

Signed-off-by: Jan Beulich <jbeulich@suse.com>