ALSA: firewire-motu: minor code refactoring to stop isochronous context

The helper function to stop isochronous context is superfluous. This
commit removes it with simpler codes.

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