dmaengine: pxa_dma: Remove an erroneous BUG_ON() in pxad_free_desc()

If pxad_alloc_desc() fails on the first dma_pool_alloc() call, then
sw_desc->nb_desc is zero.
In such a case pxad_free_desc() is called and it will BUG_ON().

Remove this erroneous BUG_ON().

It is also useless, because if "sw_desc->nb_desc == 0", then, on the first
iteration of the for loop, i is -1 and the loop will not be executed.
(both i and sw_desc->nb_desc are 'int')

Fixes: a57e16cf ("dmaengine: pxa: add pxa dmaengine driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/c8fc5563c9593c914fde41f0f7d1489a21b45a9a.1696676782.git.christophe.jaillet@wanadoo.frSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/pxa_dma.c

@@ -722,7 +722,6 @@ static void pxad_free_desc(struct virt_dma_desc *vd)
 	dma_addr_t dma;
 	struct pxad_desc_sw *sw_desc = to_pxad_sw_desc(vd);
 
-	BUG_ON(sw_desc->nb_desc == 0);
 	for (i = sw_desc->nb_desc - 1; i >= 0; i--) {
 		if (i > 0)
 			dma = sw_desc->hw_desc[i - 1]->ddadr;