net: fix phy_string_for_interface

commit ffb0f6f4 ("treewide: Rename PHY_INTERFACE_MODE_NONE to
PHY_INTERFACE_MODE_NA") broke the phy_string_for_interface function.
Fix it.

Fixes ffb0f6f4 ("treewide: Rename PHY_INTERFACE_MODE_NONE to PHY_INTERFACE_MODE_NA")
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Cc: Marek Behún <marek.behun@nic.cz>
Cc: Stefan Roese <sr@denx.de>
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>