dmaengine: Add might_sleep() to dmaengine_synchronize()

Implementations of dmaengine_synchronize() are allowed to sleep, hence the
function must not be called to from atomic context. Add might_sleep() to
dmaengine_synchronize() to make it easier to detect non-compliant callers.

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>