pcm: file: update linked hw_ptr and appl_ptr

pcm: file: update linked hw_ptr and appl_ptr

Plugin file provides no private hw_ptr and appl_ptr but instead links
them to the slave pcm.  If the slave pcm itself changes its hw_ptr or
app_prt this needs to be done in file plugin, too.

Plugin 'plug' is such a candidate changing the hw_ptr and app_ptr in
hw_params call dependent on the automatically inserted plugins.  ALSA
unfortunately has no support for automatically updating chained
pointers.

A notification on pointer change seems to be prepared inside the
snd_pcm_set_ptr() routine via rbptr->changed(), but it is not (yet)
implemented so that we need to care for it manually.

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