net: dsa: mt7530: support setting MTU

MT7530/7531 has a global RX packet length register, which can be used
to set MTU.

Supported packet length values are 1522 (1518 if untagged), 1536,
1552, and multiple of 1024 (from 2048 to 15360).

Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Link: https://lore.kernel.org/r/20201103050618.11419-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>