alsabat: refactoring alsa capture thread

alsabat: refactoring alsa capture thread

Refactoring ALSA capture thread:
  1. Move file open/seek operations to sub function, so all file
  processes are now on a single function (read_from_pcm_loop()), so
  the structure is more reasonable, the function API is simplified
  and no need file cleanup in thread loop.
  2. Replace the wav header processing lines with a general function
  (update_wav_header()), which can be reused in other sections.
  3. Add pthread_exit() for thread to exit safely in single line mode,
  and correct comment.

Signed-off-by: Lu, Han <han.lu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>