stmmac: introduce get_stmmac_bsp_priv() helper

Create a helper to retrieve dwmac private data from a dev
pointer. This is useful in PM callbacks and driver remove.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>