ALSA: pcm: Unify playback and capture poll callbacks

The poll callbacks for playback and capture directions are doing
fairly similar but with a slight difference.  This patch unifies the
two functions into a single callback.  The advantage of this
refactoring is that the direction-specific procedures become clearer.

There should be no functional change but only the code cleanup.

Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>