net: stmmac: sun8i: force select external PHY when no internal one

The PHY selection bit also exists on SoCs without an internal PHY; if it's
set to 1 (internal PHY, default value) then the MAC will not make use of
any PHY on such SoCs.

This problem appears when adapting for H6, which has no real internal PHY
(the "internal PHY" on H6 is not on-die, but on a co-packaged AC200 chip,
connected via RMII interface at GPIO bank A).

Force the PHY selection bit to 0 when the SOC doesn't have an internal PHY,
to address the problem of a wrong default value.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: David S. Miller <davem@davemloft.net>