dwc_eth_qos: enable flow control by default

Allow autoneg to enable flow control by default.
The behavior when autoneg is off has not changed.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Jesper Nilsson <jespern@axis.com>
Acked-by: Lars Persson <larper@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>