net: dsa: tag_rtl8_4: set KEEP flag

net: dsa: tag_rtl8_4: set KEEP flag

KEEP=1 is needed because we should respect the format of the packet as
the kernel sends it to us. Unless tx forward offloading is used, the
kernel is giving us the packet exactly as it should leave the specified
port on the wire. Until now this was not needed because the ports were
always functioning in a standalone mode in a VLAN-unaware way, so the
switch would not tag or untag frames anyway. But arguably it should have
been KEEP=1 all along.

Co-developed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260408-realtek_fixes-v1-2-915ff1404d56@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>