dmaengine: ppc4xx: remove xor_hw_desc assignment without reading

The xor_hw_desc local variable is assigned but never read:

    drivers/dma/ppc4xx/adma.c: In function ‘ppc440spe_desc_set_src_mult’:
    drivers/dma/ppc4xx/adma.c:562:17: warning: variable ‘xor_hw_desc’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201019155756.21445-2-krzk@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>