wan/fsl_ucc_hdlc: info leak in uhdlc_ioctl()

There is a 2 byte struct whole after line.loopback so we need to clear
that out to avoid disclosing stack information.

Fixes: c19b6d24

 ('drivers/net: support hdlc function for QE-UCC')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>