ALSA: bebob: change type of substream counter from atomic_t to unsigned int

The counter is incremented/decremented in critical section protected with
mutex. Therefore, no need to use atomic_t.

This commit changes the type to unsigned int.

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