Add support for Byte Queue Limits to the STMicro MAC driver.
Tested on a Amlogic S802 quad Cortex-A9 board, where the use of BQL
decreases the latency of a high priority ping from ~12ms to ~1ms when
the 100Mbit link is saturated by 20 TCP streams.
Signed-off-by: Beniamino Galvani <b.galvani@gmail.com> Reviewed-by: Eric Dumazet <edumazet@google.com> Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com> Signed-off-by: David S. Miller <davem@davemloft.net>