[RFKILL]: Add support for hardware-only rfkill buttons

Buttons that work directly on hardware cannot support
the "user_claim" functionality. Add a flag to signal
this and return -EOPNOTSUPP in this case.
b43 is such a device.

Signed-off-by: Michael Buesch <[e-mail]>
Acked-by: Ivo van Doorn <[e-mail]>
Signed-off-by: John W. Linville <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/rfkill.h

···
  * @type: Radio type which the button controls, the value stored
  *	here should be a value from enum rfkill_type.
  * @state: State of the switch (on/off).
+ * @user_claim_unsupported: Whether the hardware supports exclusive
+ *	RF-kill control by userspace. Set this before registering.
  * @user_claim: Set when the switch is controlled exlusively by userspace.
  * @mutex: Guards switch state transitions
  * @data: Pointer to the RF button drivers private data which will be
···
 	enum rfkill_type type;
 
 	enum rfkill_state state;
+	bool user_claim_unsupported;
 	bool user_claim;
 
 	struct mutex mutex;

net/rfkill/rfkill.c

···
 	if (error)
 		return error;
 
+	if (rfkill->user_claim_unsupported) {
+		error = -EOPNOTSUPP;
+		goto out_unlock;
+	}
 	if (rfkill->user_claim != claim) {
 		if (!claim)
 			rfkill_toggle_radio(rfkill,
···
 		rfkill->user_claim = claim;
 	}
 
+out_unlock:
 	mutex_unlock(&rfkill_mutex);
 
-	return count;
+	return error ? error : count;
 }
 
 static struct device_attribute rfkill_dev_attrs[] = {