sysctl: sysctl_binary.c Fix compilation when !CONFIG_NET

dev_get_by_index does not exist when the network stack is not
compiled in, so only include the code to follow wild card paths
when the network stack is present.

I have shuffled the code around a little to make it clear
that dev_put is called after dev_get_by_index showing that
there is no leak.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>