staging: cxt1e1: buffer overflow in do_del_chan()

If we don't restrict "cp.channum" to 3 digits then the sprintf() will
overflow.  I've added a check and changed the sprintf() to snprintf().

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