ARM: OMAP1: Fix pm_idle during suspend

Commit 9ccdac36 ([ARM] idle:
clean up pm_idle calling, obey hlt_counter) removed a check
for NULL pm_idle.

Replace the NULL assignment in the OMAP1 code with disable_hlt()
to be in sync with the core code and restore the intended behavior.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Acked-by: Tony Lindgren <tony@atomide.com>