stmmac: move hw init in the probe (v2)

This patch moves the MAC HW initialization and
the HW feature verification from the open to the probe
function as D. Miller suggested.
So the patch actually reorganizes and tidies-up some parts of
the driver and indeed fixes some problem when tune its HW features.
These can be overwritten by looking at the HW cap register at
run-time and that generated problems.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Reviewed-by: Francesco Virlinzi <francesco.virlinzi@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>