net: ks8851-ml: Fix 16-bit IO operation

The Micrel KSZ8851-16MLLI datasheet DS00002357B page 12 states that
BE[3:0] signals are active high. This contradicts the measurements
of the behavior of the actual chip, where these signals behave as
active low. For example, to read the CIDER register, the bus must
expose 0xc0c0 during the address phase, which means BE[3:0]=4'b1100.

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>