ALSA: pcm: Use the common error path in __snd_pcm_lib_xfer()

An open-coded error path in __snd_pcm_lib_xfer() can be replaced with
the simple goto to the common error path.  This also makes the error
handling more consistent, i.e. when some samples have been already
processed, return that size instead of the error code.

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