ASoC: soc-generic-dmaengine-pcm: fix kernel panic caused by no dma channel.

if there is no dma channel for sound, it will cause kernel panic.
CallTrace:
[    3.051899] Unable to handle kernel NULL pointer dereference at virtual address 00000268
[    3.056756] PC is at dmaengine_pcm_new+0x94/0x268
[    3.057189] LR is at soc_new_pcm+0x394/0x42c
[    3.225565] [<ffffff80088c8b80>] dmaengine_pcm_new+0x94/0x268
[    3.226083] [<ffffff80088c6cf0>] soc_new_pcm+0x394/0x42c
[    3.226568] [<ffffff80088bb6bc>] snd_soc_register_card+0x9cc/0xe78
[    3.227124] [<ffffff80088c733c>] devm_snd_soc_register_card+0x4c/0x88
[    3.227704] [<ffffff80088d4848>] asoc_simple_card_probe+0x3c8/0x3f8
[    3.228274] [<ffffff80084f811c>] platform_drv_probe+0x58/0xa4
[    3.228791] [<ffffff80084f638c>] driver_probe_device+0x118/0x2ac
[    3.229333] [<ffffff80084f666c>] __device_attach_driver+0x88/0x98
[    3.229886] [<ffffff80084f479c>] bus_for_each_drv+0x7c/0xac
[    3.230391] [<ffffff80084f61c0>] __device_attach+0xa4/0x124
[    3.230896] [<ffffff80084f67d8>] device_initial_probe+0x10/0x18
[    3.231425] [<ffffff80084f5820>] bus_probe_device+0x2c/0x8c
[    3.231930] [<ffffff80084f5c94>] deferred_probe_work_func+0x74/0xa0
[    3.232500] [<ffffff80080bac00>] process_one_work+0x228/0x3f4
[    3.233018] [<ffffff80080bb618>] worker_thread+0x304/0x418
[    3.233512] [<ffffff80080bff84>] kthread+0xe8/0xf0
[    3.233944] [<ffffff80080832a0>] ret_from_fork+0x10/0x30

Fixes: 57fae85633 ("ASoC: rockchip: add support for "rockchip,force-iram"")
Change-Id: Ia225ec129d04690fe64a61f15ad6152a71927d27
Signed-off-by: Wenping Zhang <wenping.zhang@rock-chips.com>

sound/soc/soc-generic-dmaengine-pcm.c

@@ -278,7 +278,7 @@ static int dmaengine_pcm_new(struct snd_soc_pcm_runtime *rtd)
 		return 0;
 
 	dev = dmaengine_dma_dev(pcm, substream);
-	if (dev->of_node) {
+	if (dev && dev->of_node) {
 		ret = of_property_read_bool(dev->of_node,
 					    "rockchip,force-iram");
 		if (ret)