ALSA: Clean up snd_device_*() codes

A few code cleanups and optimizations.  In addition, drop
snd_device_disconnect() that isn't used at all, and drop the return
values from snd_device_free*().

Another slight difference by this change is that now the device state
will become always SNDRV_DEV_REGISTERED no matter whether dev_register
ops is present or not.  It's for better consistency.  There should be
no impact for the current tree, as the state isn't checked.

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