pcm: file: Add htimestamp operation in plugin file

PCM operation htimestamp is not implemented in plugin file.
Calling snd_pcm_htimestamp() on a plugin file crashes. This scenario
is considered now.

Signed-off-by: Andreas Pape <apape@de.adit-jv.com>
Signed-off-by: Joshua Frkuska <joshua_frkuska@mentor.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>