switch: clean up robo switch driver

 * remove use_et, all supported drivers are using mii ioctls
 * remove robo.phy_addr, phy_addr was always set to ROBO_PHY_ADDR.
 * remove support for old kernel versions
 * do_ioctl is allways called with buff == NULL
 * use if_mii

SVN-Revision: 33002