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.fr
Signed-off-by: Vinod Koul <vkoul@kernel.org>