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 <[e-mail]>
Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Fixes: 646262c71aca ("wifi: mac80211: remove debugfs dir for virtual monitor")
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/driver-ops.c

···
 	 * The virtual monitor interface doesn't get a debugfs
 	 * entry, so it's exempt here.
 	 */
-	if (sdata != local->monitor_sdata)
+	if (sdata != rcu_access_pointer(local->monitor_sdata))
 		ieee80211_debugfs_recreate_netdev(sdata,
 						  sdata->vif.valid_links);