ALSA: fix section mismatch in the sound_core

The __exit cleanup_oss_soundcore() is called from
the __init init_soundcore(). This causes section mismatch
and breaks kernel's linking on sparc64.

Remove the __exit attribute from the cleanup_oss_soundcore().

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>