dmaengine: jz4780: Kill tasklets before unregistering the device

Tasklets may have been scheduled as a result of an earlier interrupt
that could still be running. Kill them before unregistering the
device.

Signed-off-by: Alex Smith <alex.smith@imgtec.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Cc: dmaengine@vger.kernel.org
Signed-off-by: Vinod Koul <vinod.koul@intel.com>