net: stmmac: rework mac_interface and phy_interface documentation

net: stmmac: rework mac_interface and phy_interface documentation

Based on new research, it has come to light that the comment that I
added in a014c35556b9 ("net: stmmac: clarify difference between
"interface" and "phy_interface"") is not fully correct.

Update the comment to properly describe the difference between the two.

All of the DTS files in the kernel tree do not mention the "mac-mode"
property, which results in mac_interface and phy_interface being the
same. Also, none of the platform glue drivers set mac_interface to
anything but PHY_INTERFACE_MODE_NA. This means that for all the
platforms known to mainline, mac_interface is either the same as
phy_interface, or it is PHY_INTERFACE_MODE_NA.

Thus, updating the definition for mac_interface in stmmac.h has no
material effect on current uses known to mainline, but the change opens
the door to cleaning up all uses.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1uytpB-00000006H23-0pRi@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>