media: firewire: firedtv-avc.c: replace BUG with proper, error return

This resolves this smatch error:

drivers/media/firewire/firedtv-avc.c:602 avc_tuner_dsd() error: uninitialized symbol 'pos'.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>