libqmi-glib,message: fix 16-bit service on big endian architectures

The latest release introduces handling of 16-bit service indications. However, only the raw message data is returned from qmi_message_get_service(), leading to incorrect values on big-endian architectures. As a consequence, libqmi ultimately fails to build as the corresponding test errors out in this case.

This patch ensures the service indication is correct on all kinds of architectures.

Note: to be cherry-picked into qmi-1-34 as well