staging: pi433: return -EFAULT if copy_to_user() fails

copy_to_user() to user returns the number of bytes that weren't copied,
but we should be returning -EFAULT to the user.

Fixes: 874bcba65f9a ("staging: pi433: New driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/pi433/pi433_if.c

@@ -689,7 +689,7 @@ pi433_read(struct file *filp, char __user *buf, size_t size, loff_t *f_pos)
 	{
 		retval = copy_to_user(buf, device->rx_buffer, bytes_received);
 		if (retval)
-			return retval;
+			return -EFAULT;
 	}
 
 	return bytes_received;