When in AVB mode we need to disable flow control to prevent MAC from
pausing in TX side.

Fixes: ec6ea8e3 ("net: stmmac: Add CBS support in XGMAC2")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>