net: stmmac: Integrate XGMAC into main driver flow

Now that we have all the XGMAC related callbacks, lets start integrating
this IP block into main driver.

Also, we corrected the initialization flow to only start DMA after
setting descriptors length.

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