net: ks8851: Split out SPI specific entries in struct ks8851_net

Add a new struct ks8851_net_spi, which embeds the original
struct ks8851_net and contains the entries specific only to
the SPI variant of KS8851.

There should be no functional change.
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: David S. Miller <davem@davemloft.net>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: Petr Stetiar <ynezz@true.cz>
Cc: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>