[ALSA] ALSA's struct _snd_pcm_substream: Obsolete open_flag

PCM Midlevel,ALSA<-OSS emulation,USB USX2Y
This patch removes open_flag from struct _snd_pcm_substream.
All of its uses are substituted by querying struct _snd_pcm_substream's
member ffile instead.

Signed-off-by: Karsten Wiese <annabellesgarden@yahoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>