HSI: omap_ssi_port: prepare start_tx/stop_tx for blocking pm_runtime calls

ssi_start_tx and ssi_stop_tx may be called from atomic context. Once
pm_runtime_irq_safe() is removed for omap-ssi, this will fail, due
to blocking pm_runtime_*_sync() calls.

This fixes ssi_stop_tx by using non-sync API and ssi_start_tx by
using a worker thread.

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

drivers/hsi/controllers/omap_ssi.h

@@ -79,6 +79,7 @@ struct omap_ssm_ctx {
  * @pio_tasklet: Bottom half for PIO transfers and events
  * @flags: flags to keep track of states
  * @wk_refcount: Reference count for output wake line
+ * @work: worker for starting TX
  * @sys_mpu_enable: Context for the interrupt enable register for irq 0
  * @sst: Context for the synchronous serial transmitter
  * @ssr: Context for the synchronous serial receiver
@@ -103,6 +104,7 @@ struct omap_ssi_port {
 	bool			wktest:1; /* FIXME: HACK to be removed */
 	unsigned long		flags;
 	unsigned int		wk_refcount;
+	struct work_struct	work;
 	/* OMAP SSI port context */
 	u32			sys_mpu_enable; /* We use only one irq */
 	struct omap_ssm_ctx	sst;