HSI: omap_ssi: drop pm_runtime_irq_safe

pm_runtime_irq_safe increases the parents runtime usage
counter effectively keeping the OMAP SoC from idling.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Tested-by: Pavel Machek <pavel@ucw.cz>