sysmocom/default.conf: Add missing default value for force_ul_amplitude

Change-Id: I04218dca21f27e32d7c85a74a3e9bbf63b499357
diff --git a/sysmocom/defaults.conf b/sysmocom/defaults.conf
index d491548..69d9ed9 100644
--- a/sysmocom/defaults.conf
+++ b/sysmocom/defaults.conf
@@ -182,6 +182,7 @@
   freq_offset: 0
   log_all_level: warning
   log_nas_level: warning
+  force_ul_amplitude: 0
 
 amarisoftue:
   tx_gain: 89