[ALSA] es1968 - Fix stuttering capture

Looks like the buffer size for the stereo capture has to be a
power of two.  Now added a constraint to buffer bytes.
Also removed unnecessary #if 0 lines.

Signed-off-by: Rene Herman <rene.herman@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>