net: ks8851: Factor out TX work flush function

While the SPI version of the KS8851 requires a TX worker thread to pump
data via SPI, the parallel bus version can write data into the TX FIFO
directly in .ndo_start_xmit, as the parallel bus access is much faster
and does not sleep. Factor out this TX work flush part, so it can be
overridden by the parallel bus driver.

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>