mac80211: Update to version 5.2-rc7

mac80211: Update to version 5.2-rc7

* Update the nl80211.h file in iw to match backports version.
* Remove the two backports from kernel 4.9, they were needed for mt76,
  but that can use the version from backports now, otherwise they
  collide and cause compile errors.
* ACCESS_ONCE() was removed in kernel 4.15, use READ_ONCE() to fix
  compile with kernel 4.19.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>