ALSA: firewire-lib: handle the case that empty isochronous packet payload for CIP

ALSA: firewire-lib: handle the case that empty isochronous packet payload for CIP

Two quadlets are at least included in isochronous packet payload for
Common Isochronous Packet (CIP) format in IEC 61883-1. However, it's
better to equip ALSA IEC 61883-1/6 packet streaming engine for contrary
packet.

This commit handles isochronous cycle to process such packet so that the
cycle is skipped.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210518130048.146596-3-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>