iface-modem-location: fix zeroing capabilities on reinitialization

The modem location interface loads modem location capabilities only during
the very first initialization of this interface.

On each later reinitialization it skips loading these capabilities from
modem, however it still goes through the whole interface initialization
state machine.

This state machine uses its own "ctx->capabilities" field for temporary
capabilities storage, which is set to MM_MODEM_LOCATION_SOURCE_NONE at the
start of the initialization process.

However, if we aren't actually loading modem location capabilities because
we're only reinitializing the interface we still need to set this
"ctx->capabilities" field to the existing (previously loaded) modem
location capabilities.

Otherwise the next interface initialization step (validation) will
permanently disable the whole location interface.

src/mm-iface-modem-location.c

@@ -1746,6 +1746,7 @@ interface_initialization_step (GTask *task)
 {
     MMIfaceModemLocation *self;
     InitializationContext *ctx;
+    MMModemLocationSource existing_capabilities;
 
     /* Don't run new steps if we're cancelled */
     if (g_task_return_error_if_cancelled (task)) {
@@ -1765,8 +1766,10 @@ interface_initialization_step (GTask *task)
         /* Location capabilities value is meant to be loaded only once during
          * the whole lifetime of the modem. Therefore, if we already have it
          * loaded, don't try to load it again. */
-        if (!mm_gdbus_modem_location_get_capabilities (ctx->skeleton) &&
-            MM_IFACE_MODEM_LOCATION_GET_INTERFACE (self)->load_capabilities &&
+        existing_capabilities = mm_gdbus_modem_location_get_capabilities (ctx->skeleton);
+        if (existing_capabilities != MM_MODEM_LOCATION_SOURCE_NONE) {
+            ctx->capabilities = existing_capabilities;
+        } else if (MM_IFACE_MODEM_LOCATION_GET_INTERFACE (self)->load_capabilities &&
             MM_IFACE_MODEM_LOCATION_GET_INTERFACE (self)->load_capabilities_finish) {
             MM_IFACE_MODEM_LOCATION_GET_INTERFACE (self)->load_capabilities (
                 self,