realtek: fix egress port mask on rtl839x

The flag to enable the outgoing port mask is in CPU header bit 43, with
bit 0 being the leftmost bit of the header. This corresponds to BIT(4)
in the third 16-bit value of the header.

Correctly set AS_DPM by fixing the off-by-one error.

Fixes: 9eab76c ("realtek: Improve TX CPU-Tag usage")
Tested-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Signed-off-by: Sander Vanheule <sander@svanheule.net>

target/linux/realtek/files-5.10/drivers/net/ethernet/rtl838x_eth.c

@@ -126,7 +126,7 @@ static void rtl839x_create_tx_header(struct p_hdr *h, int dest_port, int prio)
 			h->cpu_tag[4] = BIT(dest_port) >> 16;
 			h->cpu_tag[5] = BIT(dest_port) & 0xffff;
 		}
-		h->cpu_tag[2] |= BIT(5); // Enable destination port mask use
+		h->cpu_tag[2] |= BIT(4); // Enable destination port mask use
 		h->cpu_tag[2] |= BIT(8); // Enable L2 Learning
 		// Set internal priority and AS_PRIO
 		if (prio >= 0)