net: stmmac: dwmac-meson8b: only keep struct device around

Nothing in the dwmac-meson8b driver (except .probe itself) requires the
platform_device anymore after .probe has finished. Replace it with a
pointer to struct device since this is what the functions inside the
driver are actually accessing.
No functional changes.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>