snd_unit: handle short write as EIO
snd_unit: handle short write as EIO

At present, internal API to write to ALSA hwdep character device is just
used by HinawaSndEfw since corresponding feature is implemented to ALSA
fireworks driver (snd-fireworks). The implementation doesn't brings
short write. However, the write operation may be used by the other
drivers for future.

This commit handles the case of short write as EIO.

Signed-off-by: Takashi Sakamoto <d5ec7d0cd8074757a1f5eb708bc105c3065a65ea@sakamocchi.jp>

takaswie/libhinawa,takaswie/libhinawa