ALSA: pcm: Simplify forward/rewind codes

Factor out the common codes in snd_pcm_*_forward() and *_rewind()
functions to simplify the codes.  No functional changes.

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