powerpc: Make IRQ_NOREQUEST last to clear, first to set

When creating an irq, don't allow a concurent driver request until
we have caled map, which will likley call set_chip_and_handler to
change the irq_chip and its operations.

Similarly, when tearing down an IRQ, make sure no new uses come
along while we change the irq back to the nop chip and then reset
the descriptor to freed status.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>