ALSA: firewire-lib: use protocol error when detecting wrong value in CIP header

When detecting zero in 'dbs' field of CIP header, this packet streaming
should be aborted because of avoiding division-by-zero. This is an error
in an aspect of IEC 61883-1, thus protocol error.

This commit use EPROTO instead of EIO. Actually, the returned value is
not used for userspace and this commit has no effect.

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