firewire: fix build failure due to missing module license

The added KUnit test has no MODULE_LICENSE even if built for tristate. It
brings build failure in linux-next integration.

This commit releases the test under GPL and fixes the bug.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/lkml/20230530122450.1603af75@canb.auug.org.au/
Fixes: dc7c5163 ("firewire: add KUnit test to check layout of UAPI structures")
Link: https://lore.kernel.org/r/20230530102532.56386-1-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>