net: ks8851-ml: Fix 16-bit data access

The packet data written to and read from Micrel KSZ8851-16MLLI must be
byte-swapped in 16-bit mode, add this byte-swapping.

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>