net: stmmac: revert "support future possible different internal phy mode"

Since internal phy-mode is reserved for non-xMII protocol we cannot use
it with dwmac-sun8i.
Furthermore, all DT patchs which comes with this patch were cleaned, so
the current state is broken.
This reverts commit 1c2fa5f8 ("net: stmmac: support future possible different internal phy mode")

Fixes: 1c2fa5f8

 ("net: stmmac: support future possible different internal phy mode")
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>