firewire: add Kconfig help on building both stacks

Alas that won't work so good, because nobody reads help texts.

I thought about adding some crude multiple choice selection (build the
old stack, build the new stack, build both stacks).  It's possible, but
it would introduce awkward dummy config variables.

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