net: dlink: fix multicast stats being counted incorrectly

`McstFramesRcvdOk` counts the number of received multicast packets, and
it reports the value correctly.

However, reading `McstFramesRcvdOk` clears the register to zero. As a
result, the driver was reporting only the packets since the last read,
instead of the accumulated total.

Fix this by updating the multicast statistics accumulatively instaed of
instantaneously.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Tested-on: D-Link DGE-550T Rev-A3
Signed-off-by: Yeounsu Moon <yyyynoom@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250823182927.6063-3-yyyynoom@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Cruzer-S

drivers/net/ethernet/dlink/dl2k.c

@@ -1099,7 +1099,7 @@ get_stats (struct net_device *dev)
 	dev->stats.rx_bytes += dr32(OctetRcvOk);
 	dev->stats.tx_bytes += dr32(OctetXmtOk);
 
-	dev->stats.multicast = dr32(McstFramesRcvdOk);
+	dev->stats.multicast += dr32(McstFramesRcvdOk);
 	dev->stats.collisions += dr32(SingleColFrames)
 			     +  dr32(MultiColFrames);