Staging: vt6655-6: check keysize before memcpy()

We need to check the we don't copy too much memory.  This comes from a
copy_from_user() in the ioctl.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>