ALSA: line6: Consolidate PCM stream buffer allocation and free

The PCM stream buffer allocation and free are identical for both
playback and capture streams.  Provide single helper functions.
These are used only in pcm.c, thus they can be even static.

Tested-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>