net: dsa: microchip: enable ksz9893 via i2c in the ksz9477 driver

The KSZ9893 3-Port Gigabit Ethernet Switch can be controlled via SPI,
I²C or MDIO (very limited and not supported by this driver). While there
is already a compatible entry for the SPI bus, it was missing for I²C.

Signed-off-by: Helmut Grohne <helmut.grohne@intenta.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>