Add the missing unlock before return from at_xdmac_tasklet().

Fixes: e77e5619 ("dmaengine: at_xdmac: Fix race over irq_status")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20220107024047.1051915-1-yangyingliang@huawei.comSigned-off-by: Vinod Koul <vkoul@kernel.org>