ALSA: fireface: obsolete usage of ktime_set() for zero assignment

In development period for Linux v4.10, ktime_t became an alias of s64,
instead of union. I forgot it. We can just assign zero, instead of usage
of ktime_set(0, 0).

Fixes: 19174295 ("ALSA: fireface: add transaction support")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>