dmaengine: xgene-dma: use dma_pool_zalloc

We should use shiny new dma_pool_zalloc instead of
dma_pool_alloc/memset

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>