net: ethernet: dwmac-sun8i: Use the correct function in exit path

When PHY is not powered, the probe function fail and some resource are
still unallocated.
Furthermore some BUG happens:
dwmac-sun8i 5020000.ethernet: EMAC reset timeout
------------[ cut here ]------------
kernel BUG at /linux-next/net/core/dev.c:9844!

So let's use the right function (stmmac_pltfr_remove) in the error path.

Fixes: 9f93ac8d

 ("net-next: stmmac: Add dwmac-sun8i")
Cc: <stable@vger.kernel.org> # v4.15+
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>