ALSA: ALSA: ctxfi - Release PCM resources at each prepare call

The prepare callback can be called multiple times, thus it needs to
release and acquire the resource again by itself at the second or later
call.

Simply add pcm_release_resources() at the beginning of each prepare
callback in ctatc.c.

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