tegra20-apb-dma: remove useless use of lock

Accordingly to dma_cookie_status() description locking is not required.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: linux-tegra@vger.kernel.org
Signed-off-by: Vinod Koul <vinod.koul@intel.com>