[802.11] Remove redundant NULL pointer check after dereference

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/80211/net80211.c

 {
 	struct list_head *beacons = ctx->beacons;
 
-	if ( ! ctx )
-		return NULL;
-
 	net80211_keep_mgmt ( ctx->dev, ctx->old_keep_mgmt );
 
 	if ( ctx->probe )