simtech: fix updating bitmask during gps disabling

During disabling of gps sources the bitmask that keeps track of them is
updated incorrectly. Instead of removing the current source, all other
sources are removed from the mask.

One problem that arises from this is, that when you enable GPS after it
has been disabled completely (e.g. by disabling all GPS sources), the
code will not send a "+CGPS=1,1" command because it incorrectly assumes
that GPS is still enabled on the device.

plugins/simtech/mm-shared-simtech.c

@@ -293,7 +293,7 @@ disable_cgps_ready (MMBaseModem  *self,
     if (error)
         g_task_return_error (task, error);
     else {
-        priv->enabled_sources &= source;
+        priv->enabled_sources &= ~source;
         g_task_return_boolean (task, TRUE);
     }
     g_object_unref (task);