dmaengine: sun6i: Free the interrupt before killing the tasklet

There's still a small window between the call to sun6i_kill_tasklet and the end
of the driver remove function where a spurious interrupt might trigger, and
start using deallocated resources.

Replace the call to synchronize_irq by a free_irq, so that we're sure that we
won't get any further interrupts when we're deallocating resources.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>