net: stmmac: xgmac: Correct RAVSEL field interpretation

RAVSEL means that only RX side is available for AVB features. As we use
both TX and RX features we need to check if RAVSEL is selected and
disable AVB if only RX side is available.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>