ALSA: firewire-lib: Fix uninitialized variable err issue

The check of error is just done for the case that CIP header is available.

This commit moves auto variable into the branch to process CIP header.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: c09010ee ("ALSA: firewire-lib: handle the case that empty isochronous packet payload for CIP")
Suggested-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210520130409.GA170303@workstation


Signed-off-by: Takashi Iwai <tiwai@suse.de>