Current implementation directly uses refcount_t to maintain the life time
of fw_node, while kref is available for the same purpose.

This commit replaces the implementation with kref.

Link: https://lore.kernel.org/r/20240801022629.31857-1-o-takashi@sakamocchi.jpSigned-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>