sysinfo: Fix scheduling of downlink SACCH information
sysinfo: Fix scheduling of downlink SACCH information

The existing algorithm (present since 2012!) failed to work
in the sole case that only *one* SACCH filling type was present.

So if you had your BTS configured to only broadcast SI5, but not
broadcast SI5ter, SI6 or any other SACCH filling, it would send
the SI5 message only once on a newly-established channel, and never
again.

The old code was working for more-than-one SACCH filling, as well
as for no SACCH filling at all.

Let's also add a NOTICE message if there is no SACCH filling available
at all.  This is highly unusual and definitely a noticeable event.

Change-Id: Ica801f9b9c118f00d9e3dc2780b3123e925f59b4
Closes: OS#3057
Related: OS#2963

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts