net: dsa: microchip: ksz9477: implement MTU configuration

This chips supports two ways to configure max MTU size:
- by setting SW_LEGAL_PACKET_DISABLE bit: if this bit is 0 allowed packed size
  will be between 64 and bytes 1518. If this bit is 1, it will accept
  packets up to 2000 bytes.
- by setting SW_JUMBO_PACKET bit. If this bit is set, the chip will
  ignore SW_LEGAL_PACKET_DISABLE value and use REG_SW_MTU__2 register to
  configure MTU size.

Current driver has disabled SW_JUMBO_PACKET bit and activates
SW_LEGAL_PACKET_DISABLE. So the switch will pass all packets up to 2000 without
any way to configure it.

By providing port_change_mtu we are switch to SW_JUMBO_PACKET way and will
be able to configure MTU up to ~9000.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20220308135857.1119028-1-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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