ARM: OMAP1: Fix pm_idle during suspend

Commit 9ccdac3662dbf3c75e8f8851a214bdf7d365a4bd ([ARM] idle:
clean up pm_idle calling, obey hlt_counter) removed a check
for NULL pm_idle.

On omap1 the system hits an equivalent of suspend during idle.
If we are suspending, we need to make sure so we don't call
omap1_pm_idle during the suspend process.

Fix this by setting the pm_idle function to a an empty
function while suspending.

Reported-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>