sgsn_libgtp: Avoid ps-paging MS on GMM Suspended state

The MS notifies movement to GMM SUSPEND state because it is for instance
handling a call and cannot use PDCH anymore. Once it releases the TCH it
will ASAP move to either dedicated mode or trigger RAU, which means it
will get out of SUSPEND state. So it doesn't make sense to try paging
the MS when in that state.

This change makes test TC_suspend_nopaging pass.

Related: OS#4616
Change-Id: Ia245899eb9f16c7f839785def4ceb721a1c3a11b