net: mdio-uclass: fix type for phy_mode_str and phy_handle_str

These global variables should both have type
  static const char * const

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>