Set the RX FIFO flush watermark lower.
According to Federico and JMicron's reply,
setting it to 16QW would be stable on most platforms.
Otherwise, user might experience packet drop issue.

CC: stable@kernel.org
Reported-by: NFederico Quagliata <federico@quagliata.org>
Fixed-by: NFederico Quagliata <federico@quagliata.org>
Signed-off-by: NGuo-Fu Tseng <cooldavid@cooldavid.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>