rt2x00: Disable link tuning in rt2500usb

In the legacy rt2570 driver the link tuner was never
really called. And now the reason has finally become
apparent: It breaks TX capabilities

As soon as the device has been associated all following
TX frames will be queued in the hardware and never transmitted
to the air. Disabling sections of the link tuner did not
have the expected result, but completely disabling the
link tuner did have the right result (Both of my rt2570 devices
came back to life).

This should fix Fedora bug: 411481

v2: Fix typos

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