net: stmmac: socfpga: re-use the `interface` parameter from platform data

The socfpga sub-driver defines an `interface` field in the `socfpga_dwmac`
struct and parses it on init.

The shared `stmmac_probe_config_dt()` function also parses this from the
device-tree and makes it available on the returned `plat_data` (which is
the same data available via `netdev_priv()`).

All that's needed now is to dig that information out, via some
`dev_get_drvdata()` && `netdev_priv()` calls and re-use it.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>