net: dsa: microchip: rename ksz_spi.c to ksz9477_spi.c

Rename ksz_spi.c to ksz9477_spi.c and update Kconfig in preparation to add
more KSZ switch drivers.

Signed-off-by: Tristram Ha <Tristram.Ha@microchip.com>
Reviewed-by: Woojung Huh <Woojung.Huh@microchip.com>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>