rfkill: Fix incorrect check to avoid NULL pointer dereference

In rfkill_register, the struct rfkill pointer is first derefernced
and then checked for NULL. This patch removes the BUG_ON and returns
an error to the caller in case rfkill is NULL.
Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Link: https://lore.kernel.org/r/20191215153409.21696-1-pakki001@umn.eduSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/rfkill/core.c

@@ -1002,10 +1002,13 @@ static void rfkill_sync_work(struct work_struct *work)
 int __must_check rfkill_register(struct rfkill *rfkill)
 {
 	static unsigned long rfkill_no;
-	struct device *dev = &rfkill->dev;
+	struct device *dev;
 	int error;
 
-	BUG_ON(!rfkill);
+	if (!rfkill)
+		return -EINVAL;
+
+	dev = &rfkill->dev;
 
 	mutex_lock(&rfkill_global_mutex);