net: stmmac: make stmmac_{probe|remove}_config_dt static

Now there's no external users of these two functions, make them static
so that there aren't any new usages of stmmac_probe_config_dt().

To avoid forward declaration, move stmmac_remove_config_dt() location.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Suggested-by: Russell King <linux@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>