firewire: nosy: char device is not seekable

Amend .open handler accordingly and remove the .llseek handler.
.llseek = NULL means no_llseek (return error) since commit 776c163b

.

The only client that uses this interface is nosy-dump in linux/tools/firewire
and it knows not to seek in this char dev.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>