[media] hackrf: move RF gain ctrl enable behind module parameter

Used Avago MGA-81563 RF amplifier could be destroyed pretty easily
with too strong signal or transmitting to bad antenna.
Add module parameter 'enable_rf_gain_ctrl' which allows enabling
RF gain control - otherwise, default without the module parameter,
RF gain control is set to 'grabbed' state which prevents setting
value to the control.
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/usb/hackrf/hackrf.c

@@ -24,6 +24,15 @@
 #include <media/videobuf2-v4l2.h>
 #include <media/videobuf2-vmalloc.h>
 
+/*
+ * Used Avago MGA-81563 RF amplifier could be destroyed pretty easily with too
+ * strong signal or transmitting to bad antenna.
+ * Set RF gain control to 'grabbed' state by default for sure.
+ */
+static bool hackrf_enable_rf_gain_ctrl;
+module_param_named(enable_rf_gain_ctrl, hackrf_enable_rf_gain_ctrl, bool, 0644);
+MODULE_PARM_DESC(enable_rf_gain_ctrl, "enable RX/TX RF amplifier control (warn: could damage amplifier)");
+
 /* HackRF USB API commands (from HackRF Library) */
 enum {
 	CMD_SET_TRANSCEIVER_MODE           = 0x01,
@@ -1451,6 +1460,7 @@ static int hackrf_probe(struct usb_interface *intf,
 		dev_err(dev->dev, "Could not initialize controls\n");
 		goto err_v4l2_ctrl_handler_free_rx;
 	}
+	v4l2_ctrl_grab(dev->rx_rf_gain, !hackrf_enable_rf_gain_ctrl);
 	v4l2_ctrl_handler_setup(&dev->rx_ctrl_handler);
 
 	/* Register controls for transmitter */
@@ -1471,6 +1481,7 @@ static int hackrf_probe(struct usb_interface *intf,
 		dev_err(dev->dev, "Could not initialize controls\n");
 		goto err_v4l2_ctrl_handler_free_tx;
 	}
+	v4l2_ctrl_grab(dev->tx_rf_gain, !hackrf_enable_rf_gain_ctrl);
 	v4l2_ctrl_handler_setup(&dev->tx_ctrl_handler);
 
 	/* Register the v4l2_device structure */