ALSA: sb8 - Fix a return code in the error path

Fixed a compile warning below:
  sound/isa/sb/sb8.c: In function ‘snd_sb8_probe’:
  sound/isa/sb/sb8.c:104: warning: ‘err’ may be used uninitialized in this function
by setting the return value correctly.

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