net: stmmac: dwmac-sun8i: Return void from PHY unpower

This is a deinitialization function that always returned zero, and that
return value was always ignored. Have it return void instead.

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>