dmaengine: k3dma: fix sparse warnings

Fix sparse warnings:
drivers/dma/k3dma.c:480:20: warning: Using plain integer as NULL pointer
drivers/dma/k3dma.c:820:1: warning: symbol 'k3_dma_pmops' was not declared. Should it be static?

Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>