realtek: fix egress port mask on rtl839x

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: 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 d9516cacb087fed7716b34b1e02ce956bb6c27f1)