net: stmmac: First Queue must always be in DCB mode

According to DWMAC databook the first queue operating mode
must always be in DCB.

As MTL_QUEUE_DCB = 1, we need to always set the first queue
operating mode to DCB otherwise driver will think that queue
is in AVB mode (because MTL_QUEUE_AVB = 0).

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>