ALSA: mixart: Use nonatomic PCM ops

Like the previous patch for VX boards, miXart device driver can be
also rewritten to use nonatomic PCM ops.  Simply spinlocks are
replaced with mutex, the tasklet code is merged into the threaded irq
handler.  Also, now mgr->msg_mutex is superfluous, so merged to
msg_lock.

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