dmaengine: xilinx_dma: fix kernel-doc style for tasklet

Commit f19a11d4 ("dmaengine: xilinx: convert tasklets to use new
tasklet_setup() API") updated driver to use new tasklet_setup() API but
missed to update the documentation for the tasklet function.

Fixes: f19a11d4

 ("dmaengine: xilinx: convert tasklets to use new tasklet_setup() API")
Acked-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/20201007083113.567559-4-vkoul@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>