stmmac: dwmac-sti: turn setup callback into a probe function

By using a few functions from stmmac_platform a proper probe
function can be created from the setup glue callback. This
makes it look more like a standard driver and prepares the
driver for further clean ups.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>