net: dsa: mv88e6xxx: Correct cmode to PHY_INTERFACE_

The switch can either take the MAC or the PHY role in an MII or RMII
link. There are distinct PHY_INTERFACE_ macros for these two roles.
Correct the mapping so that the `REV` version is used for the PHY
role.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230411023541.2372609-1-andrew@lunn.ch


Signed-off-by: Jakub Kicinski <kuba@kernel.org>