dma: at_xdmac: fix a missing check on list iterator

dma: at_xdmac: fix a missing check on list iterator

The bug is here:
	__func__, desc, &desc->tx_dma_desc.phys, ret, cookie, residue);

The list iterator 'desc' will point to a bogus position containing
HEAD if the list is empty or no element is found. To avoid dev_dbg()
prints a invalid address, use a new variable 'iter' as the list
iterator, while use the origin variable 'desc' as a dedicated
pointer to point to the found element.

Cc: stable@vger.kernel.org
Fixes: dff86944f42f1 ("dmaengine: xdmac: fix print warning on dma_addr_t variable")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Link: https://lore.kernel.org/r/20220327061154.4867-1-xiam0nd.tong@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>