ALSA: use linux/io.h to fix compile warnings

For helping to reduce Greert's regression list...
  src/sound/drivers/mtpav.c: error: implicit declaration of function 'inb'
  src/sound/drivers/mtpav.c: error: implicit declaration of function 'outb'
  ...

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