In former commits, ALSA oxfw driver handles two isochronous contexts
at the same time, except for some devices which supports one endpoint
of isochronous packet stream.

This commit renames some helper functions so that they handles duplex
streams.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>