dmaengine: fix - error: potential NULL dereference 'chan'

commit 7bb587f4 "dmaengine: add interface of dma_get_slave_channel" introduced
the above error so fix it

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>