net: stmmac: clarify difference between "interface" and "phy_interface"

Clarify the difference between "interface" and "phy_interface" in
struct plat_stmmacenet_data, both by adding a comment, and also
renaming "interface" to be "mac_interface". The difference between
these are:

 MAC ----- optional PCS ----- SerDes ----- optional PHY ----- Media
       ^                               ^
 mac_interface                   phy_interface

Note that phylink currently only deals with phy_interface.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qZq83-005tts-6K@rmk-PC.armlinux.org.uk


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