ALSA: bebob: Correction for return value of .put callback

This commit is for correction of my misunderstanding about return value of
.put callback in ALSA Control interface.

According to 'Writing ALSA Driver' (*1), return value of the callback has
three patterns; 1: changed, 0: not changed, an negative value: fatal error.

But I misunderstood that it's boolean; zero or nonzero.

*1: Writing an ALSA Driver (2005, Takashi Iwai)
http://www.alsa-project.org/main/index.php/ALSA_Driver_Documentation

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>