net: ks8851: Use 16-bit read of RXFC register

The RXFC register is the only one being read using 8-bit accessors.
To make it easier to support the 16-bit accesses used by the parallel
bus variant of KS8851, use 16-bit accessor to read RXFC register as
well as neighboring RXFCTR register.

Remove ks8851_rdreg8() as it is not used anywhere anymore.

There should be no functional change.

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>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>