OMAP3: SDTI: Prevent access to sdti writing if module is not initialized.

The function sti_channel_write_trace can be run from process and interrupt
context. It has to be completed before other sti_channel_write_trace calls.

Prevent sdti writing when SDTI module is not initialized.

Signed-off-by: Roman Tereshonkov <roman.tereshonkov@nokia.com>
Acked-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>