net: ks8851: Separate SPI operations into separate file

Pull all the SPI bus specific code into a separate file, so that it is
not mixed with the common code. Rename ks8851.c to ks8851_common.c. The
ks8851_common.c is linked with ks8851_spi.c now, so it can call the
accessors in the ks8851_spi.c without any pointer indirection.

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>