ALSA: firewire-tascam: handle error code when getting current source of clock

The return value of snd_tscm_stream_get_clock() is ignored. This commit
checks the value and handle error.

Fixes: e453df44 ("ALSA: firewire-tascam: add PCM functionality")
Cc: <stable@vger.kernel.org> # v4.4+
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20190910135152.29800-2-o-takashi@sakamocchi.jp


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