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: 874bcba6

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