iwlwifi: rtw88: rtw89: fix gcc warnings

Fix -Wno-format and unused variables warnings with gcc by adopting
(to|the) FreeBSD-specific code.

Reported by: jhb
Sponsored by: The FreeBSD Foundation
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D39673

(cherry picked from commit f621b087c00776de713944ae6561e44f83e7b541)