net: ks8851: Implement register, FIFO, lock accessor callbacks

The register and FIFO accessors are bus specific, so is locking.
Implement callbacks so that each variant of the KS8851 can implement
matching accessors and locking, and use the rest of the common code.

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>