ALSA: firewire-digi00x: use in-kernel representation for the type of 8 bits

Original code for 'DoubleOhThree' encoding was written with '__u8' type,
while the type is usually used to export something to userspace.

This commit replaces the type with 'u8'.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>