isdn: type bug in isdn_net_header()

We use len to store the return value from eth_header().  eth_header()
can return -ETH_HLEN (-14).  We want to pass this back instead of
truncating it to 65522 and returning that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>