net: ks8851: Pass device node into ks8851_init_mac()

Since the driver probe function already has a struct device *dev pointer
and can easily derive of_node pointer from it, pass the of_node pointer as
a parameter to ks8851_init_mac() to avoid fishing it out from ks->spidev.
This is the only reference to spidev in the function, so get rid of it.
This is done in preparation for unifying the KS8851 SPI and parallel bus
drivers.

No functional change.

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>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>