drivers/net/wan/hdlc_cisco: Add hard_header_len

[ Upstream commit 1a545ebe380bf4c1433e3c136e35a77764fda5ad ]

This driver didn't set hard_header_len. This patch sets hard_header_len
for it according to its header_ops->create function.

This driver's header_ops->create function (cisco_hard_header) creates
a header of (struct hdlc_header), so hard_header_len should be set to
sizeof(struct hdlc_header).

Cc: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Acked-by: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>