dmaengine: ppc4xx: remove unused variable `rval'

The variable used for returning status in
`ppc440spe_adma_dma2rxor_prep_src' function is never changed
and this function just need to return 0. Thus, the `rval' can
be removed and return 0 from `ppc440spe_adma_dma2rxor_prep_src'.

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Link: https://lore.kernel.org/r/20211114060856.239314-1-wangborong@cdjrlc.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>