net: stmmac: dwmac-sun8i: Minor probe function cleanup

Adjust the spacing and use an explicit "return 0" in the success path
to make the function easier to parse.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: David S. Miller <davem@davemloft.net>