firewire: fw-sbp2: fix parsing of logical unit directories

There is a small off-by-one bug in firewire-sbp2. This causes problems
when a device exports multiple LUN Directories. I found it when trying
to talk to a SONY DVD Jukebox.

Signed-off-by: Richard Sharpe <realrichardsharpe@gmail.com>
Acked-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de> (op. order, changelog)