dma: pl08x: Export pl08x_filter_id

Export the symbol so that it is accessible to modules.
Fixes the following error:
ERROR: "pl08x_filter_id" [sound/soc/samsung/snd-soc-s3c-dma.ko] undefined!

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>