ath9k: Use a separate TX gain table for WZR-HP-G450H

The Buffalo device WZR-HP-G450H uses the index 3 for TX gain,
which is set to the high_power table currently. Later variants
of the router use the same index, but instead refer to the
low_ob_db gain table. This is not handled in the driver since
there is no way to distinguish board revisions and the high_power
table is used (incorrectly) for the newer variants.

By default, devices based on AR9300 using the TX gain index 3 have
to use the high_power table. To make sure that WZR-HP-G450H is not
broken when the high_power table is updated, use a separate array
based on information obtained from the platform data.

The current situation where only the original variant of WZR-HP-G450H
works properly stays unchanged.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>