libertas: convert CMD_802_11_RF_TX_POWER to a direct command

And while we're at it, grab min/max TX power from the firmware and use
that to validate incoming TX power requests from WEXT.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/libertas/dev.h b/drivers/net/wireless/libertas/dev.h
index f5bb40c..560d4d3 100644
--- a/drivers/net/wireless/libertas/dev.h
+++ b/drivers/net/wireless/libertas/dev.h
@@ -253,7 +253,9 @@
 	u32 connect_status;
 	u32 mesh_connect_status;
 	u16 regioncode;
-	u16 txpowerlevel;
+	s16 txpower_cur;
+	s16 txpower_min;
+	s16 txpower_max;
 
 	/** POWER MANAGEMENT AND PnP SUPPORT */
 	u8 surpriseremoved;