iwlwifi: quieten more compiler warnings
Quieten some more (valid) gcc warnings and disable dead code.
There are more warnings, some probably a compiler problem, the
other related to firmware structs which I do not want to adjust
just locally. Leave a comment to revisit after a next driver
update.
Sponsored by: The FreeBSD Foundation
(cherry picked from commit 7db7bfe1a7b95c1acfc7861e05ee8808d448715f)