net: ks8851: Factor out bus lock handling

Pull out bus access locking code into separate functions, this is done
in preparation for unifying the driver with the parallel bus one. The
parallel bus driver does not need heavy mutex locking of the bus and
works better with spinlocks, hence prepare these locking functions to
be overridden then.

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>