net: ag71xx: remove unnecessary MTU reservation

net: ag71xx: remove unnecessary MTU reservation

commit 04b385f325080157ab1b5f8ce1b1de07ce0d9e27 upstream.

2 bytes of the MTU are reserved for Atheros DSA tag, but DSA core
has already handled that since commit dc0fe7d47f9f.
Remove the unnecessary reservation.

Fixes: d51b6ce441d3 ("net: ethernet: add ag71xx driver")
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20210218034514.3421-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>