firewire: fw-sbp2: set correct maximum payload (fixes CardBus adapters)

As far as I know, all CardBus FireWire 400 adapters have a maximum
payload of 1024 bytes which is less than the speed-dependent limit of
2048 bytes.  Fw-sbp2 has to take the host adapter's limit into account.

This apparently fixes Juju's incompatibility with my CardBus cards, a
NEC based card and a VIA based card.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Acked-by: Kristian Høgsberg <krh@redhat.com>