net: stmmac: Set Flow Control to automatic mode in the driver

By default Flow Control feature is not being enabled in stmmac.

This is a useful feature that can prevent loss of packets and now that
XGMAC already supports it (along with GMAC and QoS) it makes sense to
activate it.

Switch the module parameter to FLOW_AUTO so that Flow Control is
activated.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>