ALSA: firewire-motu: more code refactoring for MOTU data block processing layer

MOTU data block processing layer has some tracepoints events. This
commit is code refactoring to split probing the events from processing
data blocks.

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