dmaengine: uniphier-xdmac: Use readl_poll_timeout_atomic() in atomic state

The function uniphier_xdmac_chan_stop() is only called in atomic state.
Should use readl_poll_timeout_atomic() there instead of
readl_poll_timeout().

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 667b9251

 ("dmaengine: uniphier-xdmac: Add UniPhier external DMA controller driver")
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://lore.kernel.org/r/1627364852-28432-1-git-send-email-hayashi.kunihiko@socionext.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>