ALSA: core: Clean up OSS proc file management

A few minor cleanups:
- Move the call of snd_info_minor_register() into snd_info_init() so
  that we can call all proc-related stuff in a shot
- Add missing __init prefix to snd_info_minor_register()
- Return an error properly from snd_oss_info_register()
- Drop snd_info_minor_unregister() that is superfluous now

Acked-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>