subsys/mgmt/hawkbit: use `zsock_setsockopt` instead of `setsockopt`

`setsockopt` is enabled by `CONFIG_POSIX_API`, use
`zsock_setsockopt` instead, otherwise there will be a
compilation error if `CONFIG_NET_SOCKETS_SOCKOPT_TLS` is
enabled.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>