The USB command CMD_RXQTC ("RX Queue Cascade Threshold Control") tries
to access the register and is always failing when using the Apple
Ethernet adapter.
It is fixed by checking flag before sending command.
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Shivamurthy Shastri <shivamurthy.shastri@linaro.org>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>