realtek: fix egress L2 learning on rtl839x

The flag to enable L2 address learning on egress frames is in CPU header
bit 40, with bit 0 being the leftmost bit of the header. This
corresponds to BIT(7) in the third 16-bit value of the header.

Correctly set L2LEARNING by fixing the off-by-one error.

Fixes: 9eab76c8 ("realtek: Improve TX CPU-Tag usage")
Tested-by: NLuiz Angelo Daros de Luca <luizluca@gmail.com>
Signed-off-by: NSander Vanheule <sander@svanheule.net>
(cherry picked from commit d6165ea7)

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

@@ -127,7 +127,7 @@ static void rtl839x_create_tx_header(struct p_hdr *h, int dest_port, int prio)
 			h->cpu_tag[5] = BIT(dest_port) & 0xffff;
 		}
 		h->cpu_tag[2] |= BIT(4); // Enable destination port mask use
-		h->cpu_tag[2] |= BIT(8); // Enable L2 Learning
+		h->cpu_tag[2] |= BIT(7); // Enable L2 Learning
 		// Set internal priority and AS_PRIO
 		if (prio >= 0)
 			h->cpu_tag[1] |= prio | BIT(3);