The TI DP83826 PHY can operate in either MII mode or RMII mode.
By default, it is configured by straps.
It can also be configured by writing to the bit 5 of register 0x17 - RMII
and Status Register (RCSR).

When phydev->interface is rmii, rmii mode must be enabled, otherwise
mii mode must be set.
This prevents misconfiguration of hw straps.
Signed-off-by: Jérémie Dautheribes <jeremie.dautheribes@bootlin.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>