dmaengine: ppc4xx: make ppc440spe_adma_chan_list static

The ppc440spe_adma_chan_list file-scope variable is not used outside of
the unit so it can be made static.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201019155756.21445-1-krzk@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>