net: dsa: mt7530: enable MTU normalization

MT7530 has a global RX length register, so we are actually changing its
MRU.
Enable MTU normalization for this reason.

Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Acked-by: Landen Chao <landen.chao@mediatek.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20201210170322.3433-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>