ALSA: aoa: Fix I2S device accounting

i2sbus_add_dev() is supposed to return the number of probed devices,
i.e. either 1 or 0.  However, i2sbus_add_dev() has one error handling
that returns -ENODEV; this will screw up the accumulation number
counted in the caller, i2sbus_probe().

Fix the return value to 0 and add the comment for better understanding
for readers.

Fixes: f3d9478b

 ("[ALSA] snd-aoa: add snd-aoa")
Link: https://lore.kernel.org/r/20221027065233.13292-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>