ASoC: intel: Fix crash at suspend/resume without card registration

When ASoC Intel SST Medfield driver is probed but without codec / card
assigned, it causes an Oops and freezes the kernel at suspend/resume,

 PM: Suspending system (freeze)
 Suspending console(s) (use no_console_suspend to debug)
 BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
 IP: [<ffffffffc09d9409>] sst_soc_prepare+0x19/0xa0 [snd_soc_sst_mfld_platform]
 Oops: 0000 [#1] PREEMPT SMP
 CPU: 0 PID: 1552 Comm: systemd-sleep Tainted: G W 4.9.0-rc6-1.g5f5c2ad-default #1
 Call Trace:
  [<ffffffffb45318f9>] dpm_prepare+0x209/0x460
  [<ffffffffb4531b61>] dpm_suspend_start+0x11/0x60
  [<ffffffffb40d3cc2>] suspend_devices_and_enter+0xb2/0x710
  [<ffffffffb40d462e>] pm_suspend+0x30e/0x390
  [<ffffffffb40d2eba>] state_store+0x8a/0x90
  [<ffffffffb43c670f>] kobj_attr_store+0xf/0x20
  [<ffffffffb42b0d97>] sysfs_kf_write+0x37/0x40
  [<ffffffffb42b02bc>] kernfs_fop_write+0x11c/0x1b0
  [<ffffffffb422be68>] __vfs_write+0x28/0x140
  [<ffffffffb43728a8>] ? apparmor_file_permission+0x18/0x20
  [<ffffffffb433b2ab>] ? security_file_permission+0x3b/0xc0
  [<ffffffffb422d095>] vfs_write+0xb5/0x1a0
  [<ffffffffb422e3d6>] SyS_write+0x46/0xa0
  [<ffffffffb4719fbb>] entry_SYSCALL_64_fastpath+0x1e/0xad

Add proper NULL checks in the PM code of mdfld driver.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: <stable@vger.kernel.org>

sound/soc/intel/atom/sst-mfld-platform-pcm.c

@@ -771,6 +771,9 @@ static int sst_soc_prepare(struct device *dev)
 	struct sst_data *drv = dev_get_drvdata(dev);
 	struct snd_soc_pcm_runtime *rtd;
 
+	if (!drv->soc_card)
+		return 0;
+
 	/* suspend all pcms first */
 	snd_soc_suspend(drv->soc_card->dev);
 	snd_soc_poweroff(drv->soc_card->dev);
@@ -793,6 +796,9 @@ static void sst_soc_complete(struct device *dev)
 	struct sst_data *drv = dev_get_drvdata(dev);
 	struct snd_soc_pcm_runtime *rtd;
 
+	if (!drv->soc_card)
+		return;
+
 	/* restart SSPs */
 	list_for_each_entry(rtd, &drv->soc_card->rtd_list, list) {
 		struct snd_soc_dai *dai = rtd->cpu_dai;