ALSA: firewire-motu: code refactoring for condition to stop streaming

In snd_motu_stream_start_duplex() function, there're two points to stop
packet streaming, however they can be merged.  This commit merges them.

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