staging: wfx: fix copy_{to,from}_user() usage

On error, copy_to_user() returns number of bytes remaining. Driver
should return -EFAULT.

Fixes: 4f8b7fab ("staging: wfx: allow to send commands to chip")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191008094232.10014-6-Jerome.Pouiller@silabs.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wfx/debug.c

@@ -256,9 +256,8 @@ static ssize_t wfx_send_hif_msg_read(struct file *file, char __user *user_buf,
 		return context->ret;
 	// Be carefull, write() is waiting for a full message while read()
 	// only return a payload
-	ret = copy_to_user(user_buf, context->reply, count);
-	if (ret)
-		return ret;
+	if (copy_to_user(user_buf, context->reply, count))
+		return -EFAULT;
 
 	return count;
 }