net: ag71xx: remove unnecessary MTU reservation

2 bytes of the MTU are reserved for Atheros DSA tag, but DSA core
has already handled that since commit dc0fe7d4.
Remove the unnecessary reservation.

Fixes: d51b6ce4

 ("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>