ALSA: PCM: check if ops are defined before suspending PCM

BE dai links only have internal PCM's and their substream ops may
not be set. Suspending these PCM's will result in their
 ops->trigger() being invoked and cause a kernel oops.
So skip suspending PCM's if their ops are NULL.

[ NOTE: this change is required now for following the recent PCM core
  change to get rid of snd_pcm_suspend() call.  Since DPCM BE takes
  the runtime carried from FE while keeping NULL ops, it can hit this
  bug.  See details at:
     https://github.com/thesofproject/linux/pull/582


  -- tiwai ]

Signed-off-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>