Fix regression causing loss of static system-information messages

In commit 8b1a2f8cd7a81c6b8c7cdb0963dcf89de7c46100 we started to
initialize bts->si_valid to 0.  This means we are skipping the manually
configured static system information.

Instead, we have to initialize bts->si_valid to bts->si_mode_static,
i.e. start with those that are static and not to be auto-generated.

Found while developing
http://git.osmocom.org/osmo-ttcn3-hacks/tree/sysinfo

Change-Id: Iab9cc93cf6d54560a72cc393cc3721a8d10e04bf
Closes: #2367
diff --git a/src/libbsc/bsc_init.c b/src/libbsc/bsc_init.c
index 6f80958..5f41a27 100644
--- a/src/libbsc/bsc_init.c
+++ b/src/libbsc/bsc_init.c
@@ -139,8 +139,8 @@
 			ms_pwr_ctl_lvl(bts->band, bts->ms_max_power);
 	bts->si_common.cell_sel_par.neci = bts->network->neci;
 
-	/* Zero, forget the state of the SIs */
-	bts->si_valid = 0;
+	/* Zero/forget the state of the dynamically computed SIs, leeping the static ones */
+	bts->si_valid = bts->si_mode_static;
 
 	/* First, we determine which of the SI messages we actually need */