ALSA: compress: allow writes in SNDRV_PCM_STATE_PREPARED state

Allow writes in SNDRV_PCM_STATE_PREPARED state so that more
than one buffer fragment can be written from user space
before calling SNDRV_COMPRESS_START.

Signed-off-by: Eric Laurent <elaurent@google.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>