net: ks8851: Permit overridding interrupt enable register

The parallel bus variant does not need to use the TX interrupt at all
as it writes the TX FIFO directly with in .ndo_start_xmit, permit the
drivers to configure the interrupt enable bits.

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>