HSI: omap_ssi_core: remove pm_runtime_get_sync call from tasklet

We may not call pm_runtime_get_sync() from tasklet, since
it can block once pm_runtime_irq_safe is removed for omap-ssi.

Since irq can should only be created for a running device,
we assume, that the device is already running and use non-
synchronous API instead.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Tested-by: Pavel Machek <pavel@ucw.cz>