ALSA: pcm: Unify ioctl functions for playback and capture streams

Some ioctl functions are implemented individually for both playback
and capture streams although most of the codes are identical with just
a few different stream-specific function calls.  This patch unifies
these places, removes the superfluous trivial check and flattens the
call paths as a cleanup.  Meanwhile, for better readability, some
codes (e.g. xfer ioctls or forward/rewind ioctls) are factored out as
functions.

Signed-off-by: Takashi Iwai <tiwai@suse.de>