ALSA: firewire-lib: fix wrong value for FDF field as an empty packet

This commit fix out of specification about the value of FDF field in out packet
with 'no data'. This affects blocking mode.

According to IEC 61883-6, there is two way to generate AMDTP packets include no
data in blocking mode.

Way 1. an empty packet defined in IEC 61883-1
 - Size of packet is 2 quadlets.
 - The value of FDF is sfc.
 - The packet includes only CIP headers

Way 2. a special non-empty packet defined in IEC 61883-6
 - Size of packet is following to blocking mode
 - The value of FDF is 0xff. This value is 'NO-DATA'. This means 'The receiver'
   must ignore all the data in a CIP with this FDF code'.
 - The packet includes dummy data.

But current implementation is a combination of them.
 - Size of packet is 2 (way 1)
 - FDF = 0xff (way 2)
This causes BeBoB chipset cannot sound.

This patch applies Way 1.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Cc: Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>