m68k/mac: Add mutual exclusion for IOP interrupt polling

The IOP interrupt handler iop_ism_irq() is used by the adb-iop
driver to poll for ADB request completion. Unfortunately, it is not
re-entrant. Fix the race condition by adding an iop_ism_irq_poll()
function with suitable mutual exclusion.

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>