firewire: obsolete usage of *-objs in Makefile for KUnit test

Nowadays *-objs list is just for user space programs.

This commit obsolete the usage, and simplify Makefile for firewire KUnit
tests since the tests are not composite objects.

Link: https://lore.kernel.org/r/20240508105351.532693-1-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>