realtek: fix egress L2 learning on rtl839x

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: 9eab76c84e31 ("realtek: Improve TX CPU-Tag usage")
Tested-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Signed-off-by: Sander Vanheule <sander@svanheule.net>
(cherry picked from commit d6165ea75baea4f9039f3a378d55219c74b932a7)