net: ks8851: Factor out spi->dev in probe()/remove()

Pull out the spi->dev into one common place in the function instead of
having it repeated over and over again. This is done in preparation for
unifying ks8851 and ks8851-mll drivers. 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>