Revert a commit to allow providing CFLAGS and LIBS from OpenWrt package
Makefile.
This downgrades the nl80211.h to kernel 5.15 and removes FILS_CRYPTO_OFFLOAD.
This is needed to make it compatible with our patched mac80211 from
kernel 5.15
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>