wifi: mac80211: Fix sparse warning for monitor_sdata

Use rcu_access_pointer() to avoid sparse warning in
drv_remove_interface().

Signed-off-by: Alexander Wetzel <Alexander@wetzel-home.de>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202502130534.bVrZZBK0-lkp@intel.com/
Fixes: 646262c7 ("wifi: mac80211: remove debugfs dir for virtual monitor")
Link: https://patch.msgid.link/20250213214330.6113-1-Alexander@wetzel-home.de


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