ALSA: firewire-lib: fix invalid length of rx packet payload for tracepoint events

Although CIP header is handled as context header, the length of isochronous
packet includes two quadlets for its payload. In tracepoints event the
value of payload_quadlets should includes the two quadlets. But at present
it doesn't.

This commit fixes the bug.

Fixes: b18f0cfa ("ALSA: firewire-lib: use 8 byte packet header for IT context to separate CIP header from CIP payload")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>