ath5k: Add tx power calibration support

* Add tx power calibration support
* Add a few tx power limits
* Hardcode default power to 12.5dB
* Disable TPC for now

v2: Address Jiri's comments

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath5k/desc.c b/drivers/net/wireless/ath5k/desc.c
index b40a928..dc30a2b 100644
--- a/drivers/net/wireless/ath5k/desc.c
+++ b/drivers/net/wireless/ath5k/desc.c
@@ -194,6 +194,10 @@
 		return -EINVAL;
 	}
 
+	tx_power += ah->ah_txpower.txp_offset;
+	if (tx_power > AR5K_TUNE_MAX_TXPOWER)
+		tx_power = AR5K_TUNE_MAX_TXPOWER;
+
 	/* Clear descriptor */
 	memset(&desc->ud.ds_tx5212, 0, sizeof(struct ath5k_hw_5212_tx_desc));