[ALSA] fix section mismatch

Fix shared init/exit function attributes:
WARNING: sound/built-in.o(.exit.text+0x4a1): Section mismatch: reference to .init.text: (between 'alsa_card_virmidi_exit' and 'alsa_card_serial_exit')
WARNING: sound/built-in.o(.exit.text+0x4c1): Section mismatch: reference to .init.text: (between 'alsa_card_serial_exit' and 'ac97_bus_exit')

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>