wifi: iwlegacy: avoid size increase

My previous patch caused a slight code size increase when DEBUG_FS is disabled,
as Stanislaw Gruszka pointed out.

Fix this with an added IS_ENABLED() check.

Fixes: cf6b9ba1 ("wifi: iwlegacy: don't warn for unused variables with DEBUG_FS=n")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Link: https://patch.msgid.link/20250304144210.997531-1-arnd@kernel.org


Signed-off-by: Johannes Berg <johannes.berg@intel.com>