rt2x00: Rename CONFIG_DISABLE_LINK_TUNING

Rename CONFIG_DISABLE_LINK_TUNING to DRIVER_SUPPORT_LINK_TUNING
Link tuning support is not only based on EEPROM decisions, but
also if the device actually supports it.

Currently only rt2500usb doesn't support link tuning because
of hardware problems. But rt2800usb is also suspected of having
problems with link tuning.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>